FBI releases video of Navy Yard shooter before he began his attack
Surveillance video of Naval Yard shooter [AFP]

The gunman who slaughtered 12 people last week in Washington is seen darting through a navy building like a special forces operator in a chilling video released Wednesday.

The FBI also said that analysis of Aaron Alexis's medical history show that he believed he was being controlled or influenced by extremely low frequency, or ELF, electro magnetic waves.

He used a sawed-off shotgun in the September 16 massacre at the Washington Navy Yard and etched into various parts of the gun were several phrases, said Valerie , assistant director of the FBI's Washington field office.

"On the left side were the phrases, quote, 'better off this way,' closed quote and quote, 'my ELF weapon,' closed quote," she told a news conference.

Police eventually shot and killed Alexis but not before he killed 12 people.

"At this point, I can confirm that there are multiple indicators that Alexis held a delusional belief that he was being controlled or influenced by extremely low frequency or ELF electro magnetic waves," Parlave said.

"The etching of 'my ELF weapon' on the shotgun is believed to reference this," she said.

Alexis was a 34-year-old former sailor with a history of disciplinary problems and brushes with the law. He had a security pass to get into the navy complex building because he was working for a navy contractor.

Parlave also said Wednesday that etched into the barrel of the shotgun were the words "end to the torment" and etched into the right side of the shotgun receiver -- the part of the gun that houses the operating pieces -- were the words "not what y'all say."

Video released by the FBI show Alexis, wearing dark pants and a purple short-sleeved shirt, darting stealthily through empty hallways at a building of the Washington Navy Yard complex the day of the massacre.

At one point he seems to take cover at a corner in a hallway and peeks around to see what lies ahead, then takes off, walking fast. He is not seen opening fire in any part of the footage.

Another piece of video shows him walking through the front door of building 197 at the yard carrying a sort of gym bag.

Authorities continue to analyze electronic media recovered from the man's belongings but preliminary evidence gives a clue as to his intentions.

"There are indicators that Alexis was prepared to die during the attack and that he accepted death as the inevitable consequence of his actions," Parlave said.

She said there is no evidence he targeted specific individuals.

Watch the footage, posted by AFP on Wednesday, below.