A sailor and a civilian were killed in a late-night shootout involving military security forces aboard a U.S. destroyer in Virginia Monday.

The incident occurred when the ship was moored at Naval Station Norfolk, one of the largest naval installations in the world. The civilian allegedly shot the sailor and then was killed by security, according to a preliminary AP report:

The shooting happened around 11:20 p.m. Monday aboard the USS Mahan, a destroyer, base spokeswoman Terri Davis said.

Davis would not describe the circumstances of the shooting but said the scene was secure. No other injuries were reported.

Davis said that the two killed were both males but that she did not have any other information on them. She said she could not say whether the civilian had permission to be aboard the ship. No other injuries were reported.

The Mahan, a guided missile destroyer of the same class as the USS Cole, is pictured below at its mooring in Norfolk. Its armaments include multiple heavy guns, torpedoes, and the capacity to launch 90 or more Tomahawk cruise missiles—enough to level a city:

All persons seeking to board a naval ship in port, military or civilian are subjected to a brief security check by watch officers from the ship's crew. For a civilian to gain access, they typically have to be government employees or contractors carrying a military-style ID, known in the service as a "common access card," or else they must generally be an authorized guest of a crewmember.

The shooting comes about half a year after a Navy contractor killed 12 people and injured three in a mass shooting with a shotgun at the Navy Yard in Washington, D.C. According to the AP, the Navy had held worldwide internal security exercises for the entire service last month, "including an active-shooter drill at Naval Station Norfolk."

Update: Via commenter Chatham Harrison below and the AP, it now appears the initial shooter got his weapon from a watch officer on the ship:

CNN cites an anonymous "defense" source as saying it went down like this:

The civilian had some type of proper identification to get onto the base, but he breached security to get onto the destroyer, the defense official said, speaking on condition of anonymity. It's not known how he got access to the pier where the ship was docked. The pier has additional security to prevent unauthorized personnel from boarding ships, the official said.

Once on board, the civilian grabbed the service pistol of a petty officer who was standing guard, the official said. Two other security sailors arrived, and the civilian fatally shot one of them, according to the official.

Navy spokesman Jim Moir said the civilian suspect was shot and killed by Navy Security Forces.

What's confusing is that, on this account, the suspect disarmed the petty officer of the watch, whose job is to control entry onto the ship, but did not shoot the petty officer; rather, he shot another sailor who responded to the scene. More details to follow soon, most likely.

[Photo credits: U.S. Navy]