A $20,000 reward is being handed out to an unnamed Bay State resident who helped cops track down two FBI rifles allegedly stolen by a 16-year-old from Lawrence.

The FBI tonight announced the cash award "is expected to be given to a member of the public."

Andover police and the FBI are now investigating how those two rifles were stolen from a bureau vehicle parked overnight in Andover.

During a press conference announcing the arrest of a 16-year-old for the theft, Andover Police Commander Charles Heseltine would not say how the FBI SWAT vehicle was broken into, but he said other cars nearby were also burglarized and those cars were left unlocked.

The two weapons, one an M16-A1 rifle, the other a sniper rifle, were stolen between Wednesday night and Thursday morning.

Failure to secure a large capacity firearm is punishable by up to 10 years in prison and up to a $10,000 fine. Heseltine said agents and police recovered the weapons as well as magazines that were stolen. Heseltine says charges are up to the DA’s office.

The weapons were inside the unmarked bureau vehicle in anticipation of an operation Thursday morning, the FBI said yesterday.

The theft of weapons from vehicles automatically triggers an investigation from Washington D.C., the FBI said.

Heseltine said car breaks have been an ongoing problem in Andover, as well as other nearby Merrimack Valley towns for years.

The Lawrence teenager who was arrested will appear in Lawrence District Court’s juvenile session later today. An FBI forensics team is still poring over the SWAT vehicle for evidence.