If you are aware of who committed this crime and you do not report this, you are guilty of Aiding & Abetting.
Mobile-friendly - A person charged with aiding and abetting or accessory is usually not present when the crime itself is committed, but he or she has knowledge of the crime before or after the fact, and may assist in its commission through advice, actions, or financial support.
@GMKBIG : knowledge before or AFTER are key definitions. I highly suggest you consider this. Of course, whomever runs this site can turn it over to law enforcement.