18 U.S.C. 111, makes it a Federal crime or offense for anyone to forcibly assault a Federal officer while the officer is engaged in the performance of his official duties.
A person can be found guilty of the offense of assaulting a Federal officer only if all of the following facts are proved beyond a reasonable doubt:
First: That the person forcibly assaulted the person described in the indictment; Second: That the person assaulted was a Federal officer as described above, then engaged in the performance of his official duty, as charged; and Third: That the person did such acts knowingly and willfully.
It is not necessary to show that the person knew the person being forcibly assaulted was, at that time, a Federal officer carrying out an official duty so long as it is established beyond a reasonable doubt that the victim was, in fact, a Federal officer acting in the course of his duty and that the person willfully committed a forcible assault upon him.
On the other hand, the person would not be guilty of a willful assault if the evidence leaves a reasonable doubt concerning whether the person knew the victim to be a Federal officer and only acted as he did because of a reasonable, good faith belief that he needed to defend himself against an assault by a private citizen.