accessory after the fact

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An accessory who assists a lawbreaker after the commission of a crime, as to avoid arrest, trial, or punishment.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun law A person who assists or protects someone wanted by the police in connection with a crime.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a person who gives assistance or comfort to someone known to be a felon or known to be sought in connection with the commission of a felony