An accessory after the fact is a person who (1) assists someone who has committed a crime, (2) after the commission of the crime, (3) with knowledge that the person committed the crime, and (4) with the intent to help the person who committed the crime avoid arrest or punishment. The United States Code defines an accessory after the fact has someone who knows a crime has been committed and receives the offender (harbors them), or relieves, comforts, or assists the offender in order to hinder or prevent the offender’s apprehension, trial, or punishment.

An accessory after the fact is different that an accomplice to a crime, who helps with the commission of the crime. An accessory after the fact is treated as a criminal offense separate from the criminal offense of the offender being assisted, and may be prosecuted as the criminal offense of obstruction of justice.

In Massachusetts, an accessory after the fact is defined under Massachusetts General Laws Chapter 274, Section 4. According to this statute, an accessory after the fact is someone who knowingly helps a felon avoid arrest, trial, or punishment after the felony has been committed. The assistance can include actions such as hiding the felon, helping them escape, or destroying evidence. It's important to note that this applies to felonies, not misdemeanors. The punishment for being an accessory after the fact in Massachusetts can include imprisonment, with the specific term depending on the underlying felony. This is distinct from being an accomplice, which involves participating in the commission of the crime itself. At the federal level, Title 18 of the United States Code, Section 3, similarly defines an accessory after the fact and outlines penalties, which can include fines and imprisonment. The federal law applies to all federal crimes and serves as a guideline for states, but each state, including Massachusetts, has its own statutes governing accessories after the fact.