An accessory-after-the-fact is someone who assists 1) someone who has committed a crime, 2) after the person has committed the crime, 3) with knowledge that the person committed the crime, and 4) with the intent to help the person avoid arrest or punishment. An accessory after the fact may be held liable for, among other things, obstruction of justice.
In Bollenbach v. United States, 326 U.S. 607 (1946), the Supreme Court refused to apply principal liability (either as a principal in the first degree or as a principal in the second degree) to an accessory-after-the-fact. This contrasts with the ability of an accessory before the fact to have principal liability.
Burden of Proof
According to the Federal Trial Handbook, the government has the burden of proof to show that the defendant assisted the principal offender after the crime.