http://www.lectlaw.com/def/a007.htm
ACCESSORY AFTER THE FACT
Whoever, knowing that an offense has been committed, receives, relieves, comforts or assists the offender in order to hinder or prevent his apprehension, trial or punishment, is an accessory after the fact; one who knowing a felony to have been committed by another, receives, relieves, comforts, or assists the felon in order to hinder the felon's apprehension, trial, or punishment. U.S.C. 18
I suppose that if you knowingly conceal evidence, they could probably come up with a charge for that. It probably wouldn't be accessory after the fact, unless someone could prove that the witness was trying the help the perpetrator escape justice (as opposed to just not saying anything for fear of repercussions) because it appears that unless you are trying to help the perp, just not saying anything doesn't make you guilty of Accessory After the Fact.