* 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 (Adds details on words carved into shotgun, Pentagon review)
By Ian Simpson
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 were 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 into a Navy Yard parking garage in a rented blue Toyota Prius 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 dressed in dark clothing, walking down a stairway and corridors in a crouch with the weapon held at the ready.
People can be glimpsed at the end of one corridor. At one point, he aims the shotgun into a room but does not fire.
Parlave said Alexis, a government technology contractor, had been armed with the shotgun, which had a sawed-off barrel and stock, and a pistol he obtained during the shooting.
"MY ELF WEAPON"
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 in 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, an FBI timeline said.
Alexis, who acted alone, was killed by police on the third floor after exchanging fire with officers 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 use.
Alexis had sought help from two Veterans Administration hospitals for insomnia. He also told police in Rhode Island he had heard voices and vibrations sent 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, a detail the Navy disclosed on Monday. 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. "That's exactly the kind of thing that we need to look at in the review." (Editing by Scott Malone, Bernadette Baum and Andre Grenon)