This case concerns an appeal in a wrongful death action taken by appellant, Alice lone Halstead Colley and those she represents, against appellee, Teddy Ray Dyer, from a district court order dismissing the complaint upon the grounds of insufficiency of service of process. Dyer was the driver of the vehicle involved in an accident resulting in fatal injuries to his passenger, Jody Glenn Dodgion, when the vehicle struck a cable strung across the exit road from the Green River, Wyoming landfill. A heavy steel chain attached to the end of the cable came through the back window of the vehicle, striking Dodgion in the head.
Soon after filing her complaint, Colley unsuccessfully attempted to make personal service of process upon Dyer at “B & R Trailer Court, Rock Springs, Wyoming 82901.” The return on the summons indicated that the deputy sheriff was “[ujnable to locate” Dyer. Colley then pursued service of process under Wyoming’s non-resident motorist statute, Wyo.Stat. § 1-6-301 (1988). That statute provides for substitute service of process on the secretary of state and requires that notice of such service, along with a copy of the process, be served upon a defendant either personally or by certified mail to his last known address. The plaintiff must file an affidavit of compliance with the clerk of the court.1 Colley’s attorney filed an affidavit stating that notice was sent to Dyer at “B & R Trailer Court, Rock Springs, Wyoming 82901;” to an insurance adjuster in care of State Farm Insurance Company; and, to Dyer’s and Dodgion’s employer who, at the time of the accident, owned the vehicle involved in the accident.
An answer was filed by Dyer through counsel hired by the insurance company to defend him. That answer raised various affirmative defenses, including allegations that process and service of process were defective.
W.R.C.P. 12(h) affords an option as to certain defenses. The defenses of insufficiency of process and insufficiency of service of process may be asserted in a responsive pleading or made by motion. The defenses at issue were properly raised in the answer. These defenses were pre*567served throughout the proceeding as evidenced by Dyer’s pretrial memorandum and were never waived.2
Thus, the issue presented for resolution involves the question of whether the certified mailing of notice to Dyer at “B & R Trailer Court, Rock Springs, Wyoming 82901,” without any reference to trailer space # 57 where Dyer once resided, satisfies the requirements of Wyo.Stat. § 1-6-301. Before this question can be answered, the requirements of Wyo.Stat. § 1-6-301 must be defined. This exercise necessarily implicates federal due process.
In Wuchter v. Pizzutti, 276 U.S. 13, 48 S.Ct. 259, 72 L.Ed. 446 (1928), the court held that a non-resident motorist statute which contained no provision for attempted notice to a non-resident defendant violated due process. The due process standard required notice to the defendant, “so as to make it reasonably probable that he will receive actual notice.” Id. at 19, 48 S.Ct. at 260.
After Wuchter, Mullane v. Central Hanover Bank & Trust Co., 339 U.S. 306, 70 S.Ct. 652, 94 L.Ed. 865 (1950) refined the federal due process standard.
Mullane involved the sufficiency of notice by publication. Recognizing that an absent defendant does not have an absolute right to be notified of the proceedings, a qualified right emerged which placed into the balance the interest of the state. The application of the balancing test led to the conclusion that statutory publication notice was sufficient for those, “whose interests or whereabouts could not with due diligence be ascertained * * *.” Id. at 317, 70 S.Ct. at 659. Thus, the court, in effect, held that substitute service was available only upon an initial showing of a diligent effort to locate the absent defendant.
This approach was affirmed in Tulsa Professional Collection Services, Inc. v. Pope, 485 U.S. 478, 490, 108 S.Ct. 1340, 1347, 99 L.Ed.2d 565 (1988) (quoting Mennonite Bd. of Missions v. Adams, 462 U.S. 791, 798 n. 4, 103 S.Ct. 2706, 2711 n. 4, 77 L.Ed.2d 180 (1983)), and was extended to protect creditors in a probate proceeding which required the personal representative to make “ ‘reasonably diligent efforts’ ” to identify creditors of the estate, rather than relying on publication notice alone.
Since federal due process requires that a diligent effort be made to locate an absent defendant before means of substitute service become available, we must interpret Wyo.Stat. § 1-6-301 in light of this requirement.
In other jurisdictions, non-resident motorist statutes providing that notice be sent to a defendant’s last known address have been interpreted to require the plaintiff to exercise due diligence in attempting to locate the absent defendant, or in ascertaining the defendant’s last known address. The cases applying a diligence requirement have done so to comply with the Wuchter standard, i.e., that it be reasonably probable that the defendant receive actual notice. See Halliman v. Stiles, 250 Ark. 249, 464 S.W.2d 573 (1971) and Drinkard v. Eastern Airlines, Inc., 290 S.W.2d 175 (Mo.App.1956).
In two recent opinions, the Washington and Utah Supreme Courts, seeking to meet the constitutional requirements of Mul-lane, have concluded that a diligence requirement is a necessary component to their respective non-resident motorist statutes. See Carlson v. Bos, 740 P.2d 1269 (Utah 1987) and Martin v. Meier, 111 Wash.2d 471, 760 P.2d 925 (1988) (statute included a due diligence requirement).
As set out in Mullane, a balancing test must be applied which weighs the interests of Dyer against the interests of the state in order to determine the proper form of notice. The state has an interest in ensuring safe highways and streets. The legislature *568has expressed the state’s interest in making certain that its residents have available to them a local forum in which to resolve their legal disputes with non-resident motorists.3 Dyer’s interest is to receive notice of the pending action against him and to have the opportunity to defend against that action. Thus, we interpret Wyo.Stat. § 1-6-301 to implicitly require the plaintiff to exercise due diligence in attempting to locate an' absent defendant.
Knowing that diligent efforts to locate Dyer are required, we must ask what comprises the “diligent efforts” needed to satisfy federal due process.
Concerning the concept of due diligence, the Supreme Court of Utah stated:
“The diligence to be pursued and shown ... is that which is reasonable under the circumstances and not all possible diligence which may be conceived. Nor is it that diligence which stops just short of the place where if it were continued might reasonably be expected to uncover an address ... of the person on whom service is sought_ Due diligence must be tailored to fit the circumstances of each case. It is that diligence which is appropriate to accomplish the end sought and which is reasonably calculated to do so.”
Carlson, 740 P.2d at 1277 n. 13 (quoting Parker v. Ross, 117 Utah 417, 217 P.2d 373, 379 (1950)).
We apply this concept in the instant case to determine whether diligent efforts were taken to locate Dyer.
After an unsuccessful attempt to personally serve Dyer at the B & R Trailer Court, Colley resorted to Wyo.Stat. § 1-6-301 for substitute service, relying on information obtained through an investigator who, almost two years after the accident, contacted Dyer’s sister, Cherryl McVicker. McVicker informed him that a month or two after the accident, Dyer left the state and that she had not seen or heard from him since.
The record does not show that anything else was done to ascertain Dyer’s present whereabouts. The investigator and Colley were apparently content that Dyer could not be found and that his last known address was “B & R Trailer Court, Rock Springs, Wyoming 82901.” If no further investigation was undertaken, then mailing notice to this address, with or without designating space # 57, falls short of the diligence required by federal due process. Moreover, this mailing, in lieu of further efforts to ascertain Dyer’s whereabouts, did little if anything to assure that Dyer would receive notice of the pending action. The diligence requirement of due process means more than a simple ascertainment of the last known address. Diligence must be applied in locating the absent defendant. This necessarily involves an attempt to develop and exhaust leads which a person normally leaves behind in the course of living. Whether Dyer had friends and/or family (parents, brothers, sisters, ex-wives, children); where these people were located; whether they had heard from Dyer or knew of his recent whereabouts, are matters for Colley’s investigator. The record is not clear whether such efforts were undertaken. We will afford Colley the opportunity to develop this portion of the record.
The record shows that notice was sent to the insurance company which was the insurer of the vehicle owned by Dyer’s employer and which vehicle was involved in the accident. At best, such notice is evidence of due diligence. Service of notice on the insurance company is not notice to the absent defendant. The insurance company is not the real party in interest, nor a designated agent for service of notice.
Notice to the insurance company, standing alone, will not satisfy due diligence. The plaintiff has a duty to exercise due diligence as federal due process requires. The plaintiff cannot expect the insurance company to carry out that duty.4
*569This is not to say that we discourage a plaintiff from forwarding notice to an insurance company. The insurance company might be able to locate the absent defendant, and this would solve the notice problem. We merely caution plaintiffs not to rely on this notice as an exclusive means of due process.
Since the enactment of Wyo.Stat. § 1-6-301, this court has not been called to pass upon its interpretation. Hence, a plaintiff seeking to perfect process under that statute might well be led to believe that service mailed to the last known address suffices, without regard to the requirement of due diligence. We note that the requirement of due diligence was not explored in the district court. It is for this reason that the matter will be remanded for a determination of whether Colley exercised due diligence in locating the absent defendant.5 See Carlson, 740 P.2d 1269.
Reversed and remanded for further proceedings consistent with this opinion.
URBIGKIT, C.J., files a specially concurring opinion.
THOMAS, J., files a dissenting opinion.
. Wyo.Stat. § 1-6-301 states in part:
(a) The use and operation of a motor vehicle on any street or highway within Wyoming by any person upon whom service of process cannot be made within Wyoming either personally or by service upon a duly appointed resident agent is deemed an appointment of the secretary of state of Wyoming as the operator’s lawful attorney upon whom may be served all legal processes in any proceeding against him * * *. Such operation constitutes the operator's agreement that any process served in any action against him * * * has the same legal force and validity as if served upon him * * *. Service shall be made by serving a copy of the process upon the secretary of state * * *. Within ten (10) days after the date of service, notice of such service and a copy of the process shall be served upon the defendant * * * either personally or by certified mail addressed to the last known address of the defendant * * *. The plaintiff shall file with the clerk of the court in which the action is brought an affidavit that he has complied with such requirement.
. Historically, counsel would have been required to make a special appearance for the single purpose of attacking insufficiency of process or lack of in personam jurisdiction. This is no longer required. As W.R.C.P. 12 indicates, the distinction between general and special appearances has been eliminated. Counsel has the option of raising the defense by motion or by raising the defense by answer. See 5A Wright & Miller, Federal Practice and Procedure: Civil 2d § 1362 at 451 (1990).
. See n. 1, supra.
. Some jurisdictions allow service on a party’s insurer. However, such jurisdictions have statutory provisions authorizing this procedure. See Annotation, Validity of Substituted Service of Process Upon Liability Insurer of Unavailable Tortfeasor, 17 A.L.R.4th 918 (1982). Absent statutory authorization, which is the case in *569Wyoming, service of notice cannot be effected in this manner.
. Upon a determination that due diligence has been exercised, the next question is whether the “last known address” requirement of Wyo.Stat. § 1-6-301 necessitated the designation of space # 57. We adopt the view that a party may rely upon the address given to the investigating authorities. See Swift v. Leasure, 285 A.2d 428 (Del.Super.1971); Kraft v. Bahr, 256 Iowa 822, 128 N.W.2d 261 (1964); and Sorenson v. Stowers, 251 Wis. 398, 29 N.W.2d 512 (1947). In this case, the accident report shows the address as "B & R Trailer Court, Rock Springs, Wyoming 82901” without designating space #57.