Philadelphia Police Commissioner Charles Ramsey says one of the departments most powerful weapons has gone missing.

In a press conference Monday morning, Ramsey said an M16 fully-automatic assault rifle is unaccounted for following an audit. He believes it couldn't have been taken by anyone but a police officer.

"It could be an inventory issue although we are going through everything we can and we still haven't located it," Ramsey told reporters Monday. "The biggest fear, obviously, is that it was stolen by one of my own members."

The Vietnam-era M16 rifle was stored in a deadbolted and alarmed storeroom at the Philadelphia Police Training Academy in Northeast Philadelphia.

For more, visit NBC10.com

The department received the gun and 1,355 other M16s from the federal government in 2009. The guns were being converted into semi-automatic AR-15 rifles. A regularly-scheduled audit, the first since December 2012, showed that one weapon was missing.

Ramsey found out about the missing rifle this weekend. He says he has never heard of this type of incident in the department before and that police know the rifle's serial number.

Only a few officers have access to the secured room and know the keypad combination to the room where the guns are stored, according to Ramsey. He says they're locked inside large crates on top of pallets. There is no surveillance video inside the room.

Ramsey doubts anyone besides his own officers had access to the weapon.

"This was not someone who came in from the outside to take this... there is no indication of that at all."

The commissioner talked tough about what he would do if he found out one of Philly's finest took the gun.

"I guarantee you that if it's somebody that I find out took that gun that I will do everything in my power to see to it that they get time in a federal penitentiary," Ramsey said.

Ramsey said that his department alerted the Federal Bureau of Investigations (FBI) and the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) about the missing weapon.

In the meantime, the department is counting their M16 inventory and comparing the weapons to recorded serial numbers.

"As far as I'm concerned it's a missing weapon," Ramsey said.

As to why the department specifically have this type of rifle: "We're cross-training some of our officers for active shooter training," Ramsey said. "That's why we have these types of weapons to supplement what our SWAT Team can do."

He says that assault rifles account for only about two percent of the shootings in the city.

Ramsey says it's the responsibility of his department to maintain and care for the weapons.

In the wake of this incident, Ramsey said he has instructed the department's audit department to take a tally of "every single firearm that we have in our department."

"We do audits but I want a special audit done in this circumstance."

He also said that moving forward there will be video cameras trained on the storeroom.

Police officers in San Francisco are scouring the city for a semi-automatic assault rifle stolen from a police vehicle over the weekend, authorities said.

Unidentified suspects smashed one of the unmarked car's windows and stole an AR-15 rifle while officers worked nearby, Officer Albie Esparza told NBC News.

The high-powered weapon is distributed to trained police officers so that it is at hand for rapid deployment during high-risk events, Esparza said.

Stay informed with the latest headlines; sign up for our newsletter

"No resource is being spared to recover this weapon," Esparza said.

Police do not know how the suspects determined the unmarked vehicle belonged to the police department.

The AR-15 assault rifle was the weapon used in last year's movie theater shooting in Aurora, Colo.