There is a law in some states that is called The Good Samaritan Law wherein if you happen to witness a crime and do not assist the person who is the victim or report the crime when it is reasonable to do so is considered breaking that law.
You could be an accessory to the crime and also you can make a citizens arrest, but make sure you won`t be harmed by doing that. Helping the police with their enquiries would be better.
Except for child/elder abuse or neglect, I don't think there is any other law which requires a person to report a crime they have witnessed. I saw a news report about a teenager whose car went into water and he drowned. His passenger never reported it. Nothing happened.
Under the general principals of Anglo-American law a person is not obligated to report a crime - just as he/she is not obliated to help the victim of a crime or the victim of an accident. This is the law in most US States.