* Video shows gunman entering garage, building
* Video shows Alexis carrying shotgun at crouch
* No signs Alexis was targeting anyone - FBI
* Exchanged fire with police for an hour - FBI
By Ian Simpson and Phil Stewart
WASHINGTON, Sept 25 (Reuters) - The FBI released surveillance video and photos of Navy Yard shooter Aaron Alexis on Wednesday and said he believed electromagnetic waves had been controlling him for months before the rampage that killed 12 people.
There are no signs that Alexis, 34, was targeting anybody in the Sept. 16 shooting at the Navy Yard in southeast Washington, 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," Parlave told a news conference.
Surveillance video released by the FBI showed Alexis driving a rented blue Toyota Prius into a Navy Yard parking garage shortly before 8 a.m. Carrying a backpack, he then entered the Naval Sea Systems Command building, site of the shootings, through a door.
The brief video also shows Alexis, armed with a Remington shotgun and wearing dark clothing, descending a stairway and walking along corridors in a crouch position, weapon held at the ready.
People can be glimpsed at the end of one corridor. Alexis peeks around corners and, at one point, aims the shotgun into a room but does not fire.
Parlave said 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 8:16 a.m. and police received the first emergency call a minute later from the fourth floor of the building, according to an FBI timeline.
Alexis, who acted alone, was killed by police on the third floor after exchanging fire with them for an hour, Parlave said.
The shooting spree raised questions about how Alexis was able to get security clearance to enter the base, despite a history of gun misuse.
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.
At the Pentagon, Deputy Defense Secretary Ashton Carter offered details on reviews meant to identify and close security gaps revealed by the shooting. His timeline included a Defense Department-wide report to be ready in December.
Carter acknowledged surprise at how Alexis' 2007 background check failed to mention a 2004 shooting. Alexis had used a gun to blow out car tires in Seattle three years before he joined the Navy and applied for a 10-year "secret" security clearance.
"What certainly caught my eye and the secretary's eye is exactly that kind of thing: evidence that there was behavior well before the Washington Navy Yard incident," Carter said.
Hewlett-Packard Co said it had terminated its relationship with The Experts, the subcontractor that employed Alexis at the Navy Yard.
The decision was based on what the company now knew about conduct by The Experts, "including its failure to respond appropriately to Aaron Alexis' mental health issues," said a Hewlett-Packard spokesman.
Hewlett-Packard did not elaborate.
The Experts said it was disappointed by HP's decision and noted that an HP site manager closely supervised Alexis.
"The Experts had no greater insight into Alexis' mental health than HP, particularly given that an HP site manager closely supervised him, including during the events in Rhode Island," it said in a statement.