United States v. Head, 707 F.3d 1026 (8th Cir. 2013)
In order to prosecute a person for accessory after the fact, the government must prove that some other person actually committed the underlying offense. Proving only that the other person was convicted of the underlying offense is not sufficient. The defendant in the accessory prosecution must also be permitted to offer evidence that she was not aware that the principal committed the offense, or that the principal was acting in self-defense.
United States v. Graves, 143 F.3d 1185 (9th Cir. 1998)
In order to be convicted of accessory after the fact (18 U.S.C. § 3) to the crime of possession of a weapon by a convicted felon, the defendant must be shown not only to have known about the principal’s possession of the weapon, but also that the principal had a felony offense. This is true, even though the government is not required to prove that the principal (in order to be guilty of being a felon in possession of a weapon) was aware that he had a felony conviction. It is true, moreover, even though someone may be found guilty of aiding and abetting another person’s possession of a weapon by a convicted felon, even though the aider is unaware of the principal’s status.