The FBI has released surveillance video and photos of Washington Navy Yard shooter Aaron Alexis and said he believed electromagnetic waves had been controlling him for months before the incident in which he killed 12 people.

There were no signs that Mr Alexis, 34, was targeting anybody in the 16 September shooting, said Valerie Parlave, the FBI assistant director in charge of the Washington field office.

“We have found relevant communications on his electronic media, which referenced the delusional belief that he was being controlled or influenced by extremely low frequency electromagnetic waves for the past three months,” Ms Parlave told a news conference.

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Surveillance video released by the FBI showed Mr Alexis driving a rented blue Toyota Prius into a Navy Yard parking garage shortly before 8am. Carrying a backpack, he then entered the Naval Sea Systems Command building, the site of the shootings, through a door.

The brief video also shows Mr Alexis, armed with a Remington shotgun and wearing dark clothing, descending a stairway and walking along corridors in a crouched stance, with the weapon held at the ready.

People can be glimpsed at the end of one corridor. Mr Alexis peeks around corners and, at one point, aims the shotgun into a room but does not fire.

Ms Parlave said Mr Alexis, a government technology contractor, had in his possession the shotgun, which had a sawed-off barrel and stock, and a pistol he obtained during the shooting.

Scratched into the shotgun were the phrases, “End to the torment,” “Not what y'all say,” “Better off this way” and “My ELF weapon,” photos released by the FBI showed. “ELF” is believed to stand for “extremely low frequency”.

The photos also showed the backpack hanging in a bathroom stall Alexis entered before starting his rampage. He shot his first victim at 8.16am and police received the first emergency call a minute later from the fourth floor of the building, according to an FBI timeline.

Mr Alexis, who acted alone, was killed by police on the third floor after exchanging fire with them for an hour, Ms Parlave said.

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The shooting spree raised questions about how Mr Alexis was able to get security clearance to enter the base, despite a history of gun misuse.

Mr Alexis had sought help for insomnia from two Veterans Administration hospitals. He also told police in Rhode Island he had heard voices and felt vibrations through hotel room walls.

Reuters

Video: CCTV footage of Aaron Alexis with a gun