Media playback is unsupported on your device Media caption CCTV footage shows Aaron Alexis stalking the corridors inside the Navy Yard complex

The FBI has released chilling video footage of the Washington DC Navy Yard gunman prowling the corridors of the complex as he hunts for victims.

During a press conference, an FBI spokeswoman said Aaron Alexis, 34, believed he was being controlled by extremely low frequency radio waves.

He killed 12 people in the rampage on 16 September before he was shot by police in a running gun battle.

Officials say they believe he did not target any specific individuals.

Clear video footage taken from the Navy Yard's gate and at locations on site shows Alexis calmly entering the complex.

'Conspiracy theories'

Later clips show him stalking a corridor armed with a shotgun and checking if a room is empty, before running down a flight of stairs.

ELF attack is what I've been subject to for the last three months, and to be perfectly honest that is what has driven me to this Aaron Alexis

The agency also released images of a weapon Alexis used during the shooting.

He had written "my ELF weapon" - apparently referring to extremely low frequency waves - and "Better off this way!" on a Remington 870 shotgun.

A photo of his backpack, which he used to bring the weapon into the building and then left in a men's toilet, was also published by the FBI.

Valerie Parlave, head of the FBI's field office in Washington, said in Wednesday's news conference that they were still investigating Alexis' background and motivations.

But she noted the former Navy reservist had a well-documented history of mental health issues.

Ms Parlave said: "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, electromagnetic waves.

"The etching... is believed to reference these electromagnetic waves.

"In addition, a document retrieved from the electronic media stated 'ELF attack is what I've been subject to for the last three months, and to be perfectly honest that is what has driven me to this'."

Ms Parlave added: "ELF technology was a legitimate programme for naval subtonal submarine communications.

"However, conspiracy theories exist which misinterpret its application as the weaponisation of remote neural frequencies for government monitoring and manipulation of unsuspecting citizens."

As an IT subcontractor, Alexis had an entry pass to the Navy Yard complex.

Investigators say he acted alone and died on the third floor of Naval Sea Systems Command headquarters after about an hour of exchanging gunfire with officers.

The shooting has led to questions about how Alexis received the security clearance necessary to be a contractor and enter the installation and prompted calls to review the government vetting process.

Also on Wednesday, Washington Police Chief Cathy Lanier told local media a Washington DC police officer was shot in the chest while responding to the shootings but was saved by protective gear.

Another police officer was shot in the leg and is currently recovering in hospital.

"We are extremely lucky that we didn't lose a police officer in there as well," Ms Lanier said.

Image caption Aaron Alexis drives his rental car, a blue Toyota Prius, through the Washington Navy Yard main gate

Image caption Alexis enters Building 197 at 8:08 (12:08 GMT) carrying his backpack, which contains a shotgun

Image caption The backpack is left hanging on the back of a cubicle door in the fourth-floor men's bathroom

Image caption He stalks the corridor with the Remington 870 shotgun

Image caption Etchings on the left side of Alexis' shotgun read "Better off this way!" and "My ELF weapon!"

Image caption Etchings on the right side of Alexis' shotgun read "Not what yall say!"