In the early morning hours of November 9, 1991, Rosebud Reservation tribal officer Kenneth Michael Scott was returning from a work-related training seminar in New Mexico, driving a white government vehicle with official license plates and a police light bar on the roof. A few miles outside of Martin, South Dakota, beyond the limits of his tribal jurisdiction, Scott came upon Lesa Primeaux walking toward Martin because her car was stuck in a snowbank. Scott stopped his car and offered Pri-meaüx a ride to Martin, which she accepted. After they traveled a short distance, Scott turned off the highway onto a side road, ordered Primeaux out of the ear, and raped her. Primeaux filed this suit for damages against the United States under the Federal Tort Claims Act, 28 U.S.C. §§ 1346(b)(1), 2671 et seq. (the “FTCA”).
Following a bench trial, the district court1 entered judgment for the government, finding that Scott’s tortious conduct was not committed within the scope of his employment as a matter of South Dakota law. A divided panel of this court reversed and remanded, directing the district court to reconsider its decision taking into account the doctrine of apparent authority. See Primeaux v. United States, 102 F.3d 1458, 1463 (8th Cir.1996) (“Pri-meaux J ”). On remand, the district court again entered' judgment for the government, finding that Scott had not used his apparent authority as a tribal police officer to commit the rape. Primeaux appealed, and a divided panel again reversed, this time directing entry of judgment in Pri-*878meaux’s favor and remanding for calculation of her damages. See Primeaux v. United States, 149 F.3d 897, 901 (8th Cir.1998) (“Primeaux II”). We granted the government’s petition for rehearing en banc, 159 F.3d 1147 (8th Cir.1998). Concluding that apparent authority is not a basis for FTCA liability in South Dakota, and that the rape was not within the scope of Scott’s government employment, we affirm.
I.
The FTCA is a limited waiver of the federal government’s sovereign immunity, allowing claims against the United States:
for ... personal injury or death caused by the negligent or wrongful act or omission of any employee of the Government while acting within the scope of his office or employment, under circumstances where the United States, if a private person, would be liable to the claimant in accordance with the law of the place where the act or omission occurred.
28 U.S.C. § 1346(b)(1); see also 28 U.S.C. § 2672. In determining the extent of the government’s FTCA liability, “ ‘[s]cope of employment’ sets the line.” Gutierrez de Martinez v. Lamagno, 515 U.S. 417, 423, 115 S.Ct. 2227, 132 L.Ed.2d 375 (1995). In this case, the parties agree that the determination of whether Scott was acting within the scope of his employment is governed by the law of the State where the tort occurred, South Dakota. See Brown v. Armstrong, 949 F.2d 1007, 1012 n. 7 (8th Cir.1991).2 Thus,, even if state law extends a private employer’s vicarious liability to employee conduct not within the scope of employment, the government’s FTCA liability remains limited to employee conduct within the scope of employment, as defined by state law. That is the situation we encounter in this case.
In determining the extent of an employer’s vicarious liability for the torts of its employees, the Supreme Court of South Dakota is guided by the principles set forth in the Restatement (Second) of Agency. See Deuchar v. Foland Ranch, Inc., 410 N.W.2d 177, 180-81 (S.D.1987); Leafgreen v. American Family Mutual Ins. Co., 393 N.W.2d 275, 277 (S.D.1986); Primeaux I, 102 F.3d at 1462. Like other agency authorities, the Restatement recognizes that conduct within an employee’s scope of employment, and conduct relied upon by a third person as consistent with the employee’s apparent authority, are distinct bases for imposing vicarious liability on the employer/principal. The distinction is made explicit in § 219(2) of the Restatement:
(2) A master is not subject to liability, for the torts of his servants acting outside the scope of their employment, unless:
* $ * * * ij;
(d) the servant purported to act or to speak on behalf of the principal and *879there toas reliance upon apparent authority., or he was aided in accomplishing the tort by the existence of the agency relation.
(Emphasis added.) As the Supreme Court recently noted, § 219(2)(d) places apparent authority liability outside .the realm of scope of employment liability — it is “an entirely separate category of agency law.” Faragher v. City of Boca Raton, 524 U.S. 775, 118 S.Ct. 2275, 2290, 141 L.Ed.2d 662 (1998). Likewise, this court and other circuits have recognized in various 'contexts that apparent authority and .scope of employment are “two theories of vicarious liability.” Commerford v. Olson, 794 F.2d 1319, 1321 (8th Cir.1986) (applying Minnesota agency law to federal 'securities law claims); see Wright-Simmons v. City of Oklahoma City, 155 F.3d 1264, 1269-71 (10th Cir.1998) (Title VII); Costos v. Coconut Island Corp., 137 F.3d 46, 48-49 (1st Cir.1998) (Maine law). If the Supreme Court of South Dakota would follow this Restatement distinction, the panel majority erred in basing FTCA liability on the doctrine of apparent authority.
To our knowledge, the Supreme Court of South Dakota has not had occasion to apply or even cite § 219(2)(d)‘of the Restatement. The Court has long followed the agency principle “that a master cannot be held liable for his servant’s acts or negligence beyond the scope of his employment, even though the injury could not have been committed without the facilities afforded to the servant by the master.” Morman v. Wagner, 63 S.D. 547, 262 N.W. 78, 79 (1935). In Morman, the Court applied a rather general test for determining scope of employment:
[I]f the act is for the benefit of the employer, either directly or indirectly, the act is within the general scope of the servant’s employment, but if the act performed is not in any way connected with the service for which he is employed, but for his own particular and peculiar purposes, then the act is not within the scope of the employment.
262 N.W. at 79. In the later case of Alberts v. Mutual Serv. Cas. Ins. Co., 80 S.D. 303, 123 N.W.2d 96, 98-99 (S.D.1963), the Court adopted the Restatement’s mul-ti-factor test for determining whether employee conduct is within the scope of employment. That test; now found in § 229 of the Restatement, does not include the principles of apparent authority’ found elsewhere in the Restatement. Its focus is on the relationship of the employee’s conduct to the employer’s-business.3 -
The doctrine of apparent or ostensible authority has a long but independent lineage in South Dakota law. It is in large part statutory, see S.D. Codified Laws §§ 59-3-3, 59-6-3, and its focus is on the reliance of'the victim on the agent’s apparent authority to act on behalf of a principal. “Strictly speaking, osténsible agency is no agency at all; it is in reality based entirely on an estoppel.” Federal Land Bank of Omaha v. Sullivan, 430 N.W.2d 700, 701 (S.D.1988). The independent origin and application of the doctrine in early cases are strong evidence the Supreme Court of South Dakota, if faced with the issue in this case,4 would follow the Restatement’s recognition that scope of em*880ployment and apparent authority are distinct theories of vicarious liability.
Primeaux nonetheless argues that under South Dakota law the doctrine of scope of employment “includes both actual and apparent authority.” Like the panel majority, Primeaux relies for this proposition on the Supreme Court of South Dakota’s opinion in Leafgreen. In that case, the Court held that an insurance company was not vicariously liable when its former employee used information obtained while employed as defendant’s agent to burgle an insured’s home. The Court began its analysis by reciting the doctrine of apparent authority — “a principal may be held liable for fraud and deceit committed by an agent within his apparent authority, even though the agent acts solely to benefit himself.” 393 N.W.2d at 277. It then quoted portions of § 261 of the Restatement, which further refines when an agent’s apparent authority makes the principal vicariously liable to third persons for the agent’s fraud.5 Recognizing a need to limit the extent of apparent authority liability for criminal misconduct, and following the lead of courts in other jurisdictions, the Court then borrowed the concept of foreseeability found in § 231 of the Restatement, one of the provisions dealing with scope of employment. For an employer to be vicariously liable under the apparent authority doctrine, the Court concluded, “the employee’s conduct must not be so unusual or startling that it would be unfair to include the loss caused by the injury among the costs of the employer’s business.” Id. at 280-81. Applying that standard to the facts in Leafgreen, the Court held the insurer not liable for its agent’s burglary.
As subsequent South Dakota cases demonstrate, the fact that Leafgreen borrowed the scope of employment foreseeability factor in applying the doctrine of apparent authority does not mean that the Supreme Court of South Dakota now considers apparent authority to be a part of the scope of employment inquiry. Less than one year after Leafgreen, the Court decided Deuchar, where a ranch hand negligently shot a visitor to the ranch while hunting. The Court analyzed Deuchar under the scope of employment doctrine, quoting Restatement § 229 for the relevant standard. 410 N.W.2d at 180 n. 2. The Court explained that an employer is not liable “for the acts of a servant who has embarked upon a ‘frolic’ of his own with no underlying purpose of furthering his master’s business,” but the scope of employment question becomes more difficult “when a servant acts for dual purposes, that is, to serve the master and to further personal interests.” Citing Leafgreen, the Court “adopted the foreseeability test for determining when a servant’s acts are within the scope of employment.” Id. at 181.-
Leafgreen and Deuchar applied the same foreseeability test, but they did not intermingle the two distinct theories of vicarious employer liability — scope of employment and apparent authority. In two more recent decisions, the Supreme Court of South Dakota has maintained this analytical distinction. McKinney v. Pioneer Life Ins. Co., 465 N.W.2d 192, 194-95 (S.D.1991), was an insurance agent fraud case; like Leafgreen, it was analyzed under the doctrine of apparent authority. More significantly, South Dakota Public Entity Pool for Liability v. Winger, 566 N.W.2d 125, 128-29 (S.D.1997), involved an insurance coverage issue that turned on scope of employment. The Court applied the traditional scope of employment analysis of Deuchar, rather than the apparent authority analysis of Leafgreen.
*881Primeaux has failed to cite any case that imposed FTCA liability solely because the victim of an intentional tort relied upon a federal employee’s apparent authority. The Restatement distinguishes between scope of employment and apparent authority as theories of vicarious liability. South Dakota law generally follows the Restatement. The Supreme Court of South Dakota has long recognized that vicarious liability for conduct within the scope of employment is based upon the employment relationship, whereas vicarious liability for conduct within an agent’s apparent authority is based upon estoppel. Recent South Dakota cases have linked the discrete analyses with a common foreseeability element, while continuing to recognize two distinct legal. theories. In these circumstances, we agree with the government that the panel majority in Pri-meaux I and Primeaux II erred in applying the South Dakota doctrine of apparent authority to determine whether Officer Scott was acting within the scope of his employment for FTCA purposes when he raped Lesa Primeaux.
II.
The remaining question is whether the district court erred in "finding that the rape of Primeaux was not within the scope of Scott’s employment as a tribal officer, applying the traditional South Dakota scope of employment standard. At the outset, we note some uncertainty whether we should review this question under the clearly erroneous or the de novo standard’ of review.6 We put this question aside, however, because the district court’s judgment must be affirmed under either standard.
In oral findings and conclusions at the end of trial, the district court summarized the scope of employment standard in Deuchar and then compared this case with two of our prior FTCA decisions applying South Dakota law. In Lushbough, an off-duty surveyor trainee borrowed a government truck and negligently caused an accident while completing personal errands. We reversed an FTCA judgment against the United States, concluding it was “too clear for argument that at the time of the accident ... [the trainee] was not acting within the scope of his office or employment.” . 200 F.2d at 721. By contrast, in Red Elk v. United States, 62 F.3d 1102 (8th Cir.1995), an on-duty police officer detained a thirteen-year-old girl for a curfew violation and raped her in the back seat of his police car. We acknowledged that this criminal conduct was done to benefit the officér personally, not his government employer, but concluded it was foreseeable that on-duty police occasionally misuse their authority in this manner. In affirming a determination that the assault was within the scope of the police officer’s employment, we expressly distinguished Lushbough: “In Lushbough the agent borrowed the principal’s vehicle to run a purely personal errand unconnected with the principal and had an accident.... There is more in our present .case, however, than *882just using the police vehicle.” 62 F.3d at 1106.
This case, the district court reasoned, “is somewhat in the middle between Lush-bough and Red Elk.” When he raped Pri-meaux, Officer Scott was using a government car for authorized travel, but he was unarmed, out of uniform, and off duty, insofar as his law enforcement responsibilities were concerned. Indeed, he was outside his jurisdiction. Scott gave Primeaux a ride because she was stranded far from town late on a cold night — the kind of assistance any citizen might offer — not to detain her as part of his law enforcement duties. Noting that Scott’s act of sexual assault was obviously not in furtherance of his government mission, the court determined that the assault “was committed while Mr. Scott was on a frolic of his own and not acting in the course or scope of his employment.”
If scope of employment is a finding of fact, this finding is not clearly erroneous. Treating scope of employment as a mixed question of fact and law reviewable de novo, we agree with the district court’s conclusion. Red Elk and recent cases from other jurisdictions have concluded it is sufficiently foreseeable to a government employer that on-duty police officers will occasionally misuse their authority to sexually assault detainees. But this case is far different. The connection between Officer Scott’s government employment and his sexual assault of Primeaux was simply too remote and tenuous to be foreseeable to his employer. In the words of Leafgreen, it was conduct “so unusual or startling that it would be unfair to include the loss caused by the injury among the costs of the employer’s business.”
The judgment of the district court is affirmed.
. The HONORABLE RICHARD H. BATTEY, United States District Judge for the District of South Dakota.
. This proposition was open to question under the FTCA as initially enacted. See United States v. Lushbough, 200 F.2d 717, 720 (8th Cir.1952). But the Supreme Court's two-sentence remand in Williams v. United States, 350 U.S. 857, 76 S.Ct. 100, 100 L.Ed. 761 (1955), was construed as resolving the issue. In 1988, Congress enacted the Federal Employees Liability Reform and Tort Compensation Act (the Westfall Act), which amended the FTCA to provide that, in a common law tort action against a federal employee, the United States is substituted as defendant if the Attorney General certifies the employee acted "within the scope of his ... employment.” 28 U.S.C. § 2679(d)(1). This certification confers immunity on the employee personally. See H.Rep. No. 100-700 (1988), reprinted in 1988 U.S.C.C.A.N. 5945. In general, an issue of federal employee privilege or immunity is one of federal law because "[n]o subject could be one of more peculiarly federal concern.” Howard v. Lyons, 360 U.S. 593, 597, 79 S.Ct. 1331, 3 L.Ed.2d 1454 (1959). In this case. Officer Scott was not sued personally so there was no Attorney General certification, and the government does not question the applicability of South Dakota law. Therefore, we do not consider whether the Westfall Act casts doubt on the widely accepted principle that the law of the State where the alleged tort occurred governs the FTCA scope of employment issue.
. One factor is “whether or not the act is seriously criminal.” § 229(2)(j). An employee's intentional, violent crime is unlikely to be within the scope of employment, but there are exceptions. For example, in Hasche v. Wagner, 55 S.D. 595, 227 N.W. 66 (1929), the principal was liable for the resulting violent assault when he told an agent to use such force as might be necessary in repossessing farm equipment.
. This inquiry is necessarily hypothetical because federal courts have exclusive jurisdiction over FTCA cases, so the Supreme Court of South Dakota is not presented with cases where an employer’s liability is limited to scope of employment, rather than the full array of vicarious liability theories under state law. We have traditionally bridged this gap by looking to general agency principles such as the Restatement. See United States v. Farmer, 400 F.2d 107, 109-110 (8th Cir.1968).
. As the Primeaux I panel majority observed, 102 F.3d at 1462, the tortious misconduct of employee Scott in this case is more directly addressed in § 265 of the Restatement, rather than the fraud provisions in § 261. The Restatement expressly lists the apparent authority provisions of § 261 and § 265 as two of the situations "in which a master may be liable for torts of servants acting solely for their own purposes and hence not in the scope of employment.” Restatement of Agency 2d § 219, cmt. e.
. The Supreme Court of South Dakota considers scope of employment a question of fact for the jury. See Deuchar, 410 N.W.2d at 181. At least one circuit reviews FTCA scope of employment findings for clear error. See Duffy v. United States, 966 F.2d 307, 314 (7th Cir.1992). The Supreme Court called FTCA scope of employment the "determination of a fact” in Lamagno, 515 U.S. at 424, 115 S.Ct. 2227, but more recently commented in another context, "scope of employment ultimately expresses a conclusion not of fact but of law.” Faragher, 118 S.Ct. at 2287. Calling this a mixed question of fact and law still leaves the standard of review in doubt. See, e.g. Smith v. Equitable Life Assur. Society, 67 F.3d 611, 616 (7th Cir.1995), noting the Seventh Circuit has "moved decisively” toward clear error review of all mixed questions of fact and law based upon recent Supreme Court decisions such as Cooter & Gell v. Hartmarx Corp., 496 U.S. 384, 402, 110 S.Ct. 2447, 110 L.Ed.2d 359 (1990). Perhaps reflecting this confusion, the panel majority reviewed the district court's scope of employment determination for clear error in Primeaux I, 102 F.3d at 1460, but reviewed the apparent authority determination de novo in Primeaux II, 149 F.3d at 898-99, 901.