ON MOTION FOR REHEARING EN BANC
Before CHRISTIE, C.J., and HORSEY, MOORE, WALSH and HOLLAND, JJ., (constituting the Court en banc). HOLLAND, Justice:This Court granted the State’s motion for rehearing en banc. The rehearing was limited to the implications of this Court’s opinion on the standard principal/accomplice jury instruction involving a single incident, which historically has required only general unanimity, and the requirement for a specific unanimity instruction under the facts and circumstances of this case.
A general unanimity instruction is required in every criminal case. Fountain v. State, Del.Supr., 275 A.2d 251, 251-52 (1971). However, this Court does not hold that a specific unanimity instruction is required in every case where a defendant may be convicted as a principal or as an accomplice. In fact, this Court recognizes that even when principal and accomplice liability theories are advanced, a general unanimity instruction is usually sufficient in the absence of a defense request for a specific instruction or in the absence of unusual circumstances creating a potential for confusion, e.g., alternative incidents which subject the defendant to criminal liability. See Shivers v. United States, 533 A.2d 258, 261-63 (D.C.1987).
The original opinion in this case recognizes that the unanimous verdict mandate which is set forth in the United States Constitution, the Delaware Constitution, and the Superior Court Criminal Rules “requires jurors to be in substantial agreement as to just what a defendant did as a step preliminary to determining whether the defendant is -guilty of the crime charged.” United States v. Gipson, 553 F.2d 453, 457-58 (5th Cir.1977). Therefore, in cases such as the one before the Court, because of the possibility of a nonunani-mous verdict, when one count encompasses two separate incidents, the trial judge must instruct the jury that if a guilty verdict is returned, the jurors must be unanimous as to which incident they find the defendant guilty. Hack v. United States, 445 A.2d 634, 641 (D.C.1982).
The need for a specific unanimity instruction flows from the fact that the basis for liability stems from two separate incidents and not from the applicability of principal or accomplice liability with respect to one of the two incidents. The necessity for a specific unanimity instruction under a single count of an information or indictment depends upon whether each act or theory under the count involves a separately cognizable incident, e.g., “by reference to separate allegations and/or to separate defens*123es.” Scarborough v. United States, 522 A.2d 869, 873 (D.C.1987) (en banc). “There is no danger of a nonunanimous verdict when one charge encompasses only a single incident. However, when one charge involves two separate incidents, the court must instruct the jurors that they must be unanimous as to which incident they rely upon as a basis for their verdict.” Owens v. United States, 497 A.2d 1086, 1093 (D.C.1985) (emphasis added) (citations omitted), cert. denied, 474 U.S. 1085, 106 S.Ct. 861, 88 L.Ed.2d 900 (1986). The single charge against Probst was based upon two separately cognizable incidents (her shot and Miller’s shot). The trial court also concluded that each incident was subject to separate defenses.10
In this case, the jury’s identification of the incident which is the basis for liability necessarily identifies Probst’s role as a principal or an accomplice. However, the requirement of Gipson that the jury must unanimously agree upon which incident is the basis for criminal liability has been held not to mean that the jury must identify the role of the defendant in that incident as either a principal or an accomplice. United States v. Peterson, 768 F.2d 64, 67 (2d Cir.), cert. denied, 474 U.S. 923, 106 S.Ct. 257, 88 L.Ed.2d 264 (1985). We agree with that interpretation of Gipson by the United States Court of Appeals for the Second Circuit.
Delaware law currently provides that a person indicted as a principal may be convicted as an accomplice and that a person indicted as an accomplice may be convicted as a principal. 11 Del.C. § 275.11 In a case where the jury is instructed pursuant to Section 275, the jury must unanimously find that a principal-accomplice relationship existed between the participants with respect to a particular charge. There is no requirement, however, that the jury be unanimous in its view as to which of the parties was the principal and which was the accomplice.
In a criminal charge involving one incident and two people, the jury is regarded as being unanimous if, without specifically identifying who was the principal and who was the accomplice, they all agree that one of the two actors performed all of the elements of the offense charged as a principal and that both actors knowingly participated in the alleged criminal act.12 Cf. United States v. Peterson, 768 F.2d at 67; United States v. Eagle Elk, 820 F.2d 959, 961 (8th Cir.), cert. denied, — U.S. -, 108 S.Ct. 191, 98 L.Ed.2d 143 (1987). This bolding is consistent with the general principle of criminal law that one cannot be convicted as an accomplice unless the State’s proof establishes that the substantive offense was committed by *124someone. United States v. Bryan, 483 F.2d 88, 93 (3d Cir.1973) (en banc).
This Court’s concern about the need for an instruction on specific unanimity is expressed in the context of the unusual facts and circumstances of this case. Requests for instructions on the need for specific unanimity, even in a case involving principal and accomplice theories of liability, will continue to be decided by the trial court according to the factors present in each particular case. In this case, a specific unanimity instruction was desirable since there was one charge (assault) and evidence of two separate incidents (Probst’s shots and Miller’s shots) to support a conviction on alternate theories of liability. Cf. Davis v. United States, 448 A.2d 242, 243-44 (D.C.1982) (per curiam).
. Prior to the rehearing en banc in this case, the parties were asked to consider how the trial court should have supplemented its instructions if the following question had appeared in a note from the Probst jury.
Six jurors believe that Probst shot Walla. Although those same jurors do not believe that Miller shot Walla, they do believe that Miller was justified in shooting at Walla.
The remaining six jurors do not believe that Probst shot Walla. Those jurors believe that Miller shot Walla without justification.
Have we reached a verdict on the assault charge?
All parties agreed that the jury would not have reached a verdict because the jury would not be unanimous with respect to the criminality of a single incident.
. The debate about principal and accomplice theories of criminal liability has continued for many years in this State and throughout the country. Under a prior statute, this Court held that an accessory was not a principal and that there could be no conviction of a defendant as an accessory upon an indictment as a principal. Schwartz v. State, Del.Supr., 185 A. 233, 234-36 (1936). That holding has been set aside by subsequent decisions of this Court and amendments to the statute. Johnson v. State, Del.Supr., 215 A.2d 247, 250 (1965); 11 Del.C. § 275. Section 275 currently reads:
(a) A person indicted for committing an offense may be convicted as an accomplice to another person guilty of committing the offense.
(b) A person indicted as an accomplice to an offense committed by another person may be convicted as a principal.
11 Del.C. § 275.
.Therefore, in a case involving two people and a single incident, if the State cannot prove who actually caused the physical injury, it does not mean that both persons will escape criminal responsibility.