ROUGH CUT. NO REPORTER NARRATION. MUTE.

Surveillance video released by the FBI shows Washington Navy Yard shooter Aaron Alexis entering a building, running down a stairwell, and peering around corners with a gun in his hand.

Alexis believed he was being controlled by extremely low frequency radio waves, the FBI said at a news conference on Wednesday.

Alexis went on a shooting rampage at the Washington Navy Yard in southeast Washington, D.C., last week, killing 12 people. He had been working at the facility as an information technology subcontractor, using access he gained in part because of his 10-year security clearance issued in 2008.

The FBI said evidence shows that Alexis acted alone in the rampage.