United States v. White, 545 Fed. App’x 69 (2d Cir. 2013):

Under certain circumstances, an attorney who has "first-hand knowledge of the events presented at trial" may impermissibly act as an unsworn witness. United States v. Locascio, 6 F.3d 924, 933 (2d Cir. 1993); see also Ciak v. United States, 59 F.3d 296, 304-05 (2d Cir. 1995) (holding that defense counsel acted as an unsworn witness when he sought to impeach a government witness regarding statements that the government witness had previously made to counsel), abrogated in part on  [*71]  other grounds as stated in Mickens v. Taylor, 535 U.S. 162, 172 n.3, 122 S. Ct. 1237, 152 L. Ed. 2d 291 (2002).  An attorney who acts as an unsworn witness should be disqualified as counsel, in part to prevent the attorney from "subtly impart[ing] to the jury his first-hand knowledge of the events without having to swear an oath or be subject to cross examination." Locascio, 6 F.3d at 933. Without more, however, a prosecutor does not become an unsworn witness merely because he has first-hand knowledge of the events that culminated in a defendant's trial. See United States v. Regan, 103 F.3d 1072, 1083 (2d Cir. 1997)  ("Standing alone, the mere fact that a prosecutor took part in grand jury proceedings in which a defendant presented false testimony should not bar that prosecutor from participating in a subsequent trial for perjury."). A problem arises, we have suggested, where there is some "indication in the record that [the prosecutor] sought to use [his] first-hand knowledge of [the] case to influence the jury." Id.