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 analysis of Aaron Alexis's medical history shows he believed he was being controlled or influenced by extremely low frequency, or ELF, electro magnetic waves.

Alexis used a sawed-off shotgun and a handgun in the September 16 massacre at the Washington Navy Yard. Etched into various parts of the shotgun were several phrases, said Valerie Parlave, 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 the dozen people died in the latest mass shooting to shock America. Four other people were wounded.

Parlave said Alexis, who is not seen opening fire at any point in the footage, apparently had medical issues.

"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 added.

ELF wave technology is used by submarines to communicate while at considerable depth.

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

Parlave also said Wednesday that etched into the barrel of the shotgun were the words "end to the torment." Carved 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."

The FBI website says a document retrieved from Alexis' electronic media stated: "Ultra low frequency attack is what I've been subject to for the last 3 months, and to be perfectly honest that is what has driven me to this."

A 31-second segment of video released by the FBI shows Alexis, wearing loose fitting dark pants and a purple short-sleeved shirt, moving stealthily through the empty hallways of a building at the complex.

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.

At the far end of the hallway, at some distance, people are seen scurrying across from left to right as they flee.

In another section of the video he runs down a staircase and opens a glass door.

He fired his first shot at 8:16 am, the FBI said in a statement on its website. Alexis was shot and killed by police 69 minutes later.

Alexis came into the building with the Remington 870 shotgun -- barrel and stock both sawed off -- and acquired a Beretta handgun once he was inside. He used the handgun in the massacre, too.

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

Authorities continue to analyze electronic media recovered from Alexis' 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.