In a document retrieved by investigators from one of his electronic devices, Mr. Alexis, a former Navy reservist and military contractor, said that "ultralow frequency attack is what I've been subject to for the last three months," said the senior official, Valerie Parlave, the assistant director in charge of the F.B.I.'s Washington Field Office.
"To be perfectly honest, this is what has driven me to this," Mr. Alexis wrote, according to Ms. Parlave.
The phrases "my elf weapon," "end to the torment," "not what ya'll say" and "better off this way," were etched into the side of the shotgun that Mr. Alexis used to kill many of the victims, she said.
Ms. Parlave, who spoke at a news conference outside the F.B.I.'s Washington Field Office, said that E.L.F. was a technology the Navy used for submarine communication. But, she said, some conspiracy theorists believe that it is the "weaponization of remote neuro-frequencies for government monitoring and manipulation of unsuspecting citizens."
In communications obtained by the authorities, Mr. Alexis said that "he was prepared to die in the attack and accepted death as the inevitable consequence of his actions," Ms. Parlave said.
Ms. Parlave also said that investigators believed Mr. Alexis targeted his victims randomly and was not seeking to shoot his co-workers.
The F.B.I. on Wednesday also released surveillance video that shows Mr. Alexis, who was killed in a shootout with the police at the Navy Yard, arriving alone in a car at the facility's parking garage. He can be seen setting up his shotgun and walking down a hallway, ducking in and out of doorways, before opening fire.
The video, which appears to be edited to exclude images of people being shot, does not show him in the atrium area overlooking the cafeteria where several people were killed as they were getting breakfast. Mr. Alexis is seen with the shotgun that F.B.I. officials said was used in the shootings.
Also Wednesday, a senior defense official said that the Pentagon had started three separate reviews of its security procedures in response to the shooting.
The official, Ashton B. Carter, the deputy defense secretary, told reporters that the Office of the Secretary of Defense, the Navy and an independent panel were looking into potential flaws in base security, background investigations and other areas that could have allowed the Navy Yard shooting to occur.
"The bottom line is, we need to know how an employee was able to bring a weapon and ammunition onto a D.O.D. installation and how warning flags were either missed, ignored or not addressed in a timely manner," Mr. Carter said.
Mr. Carter confirmed that the department was considering including more information from law enforcement in future background checks for personnel. The secretary of the Navy, Ray Mabus, recently recommended that such checks look at all available police reports, regardless of whether they resulted in charges being filed or convictions. Mr. Alexis had two gun-related incidents in his past, and was jailed briefly in Georgia, but those incidents did not raise red flags during background checks.
"We have to confirm the truth of these things," Mr. Carter said, "but I think that 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, which, had it been spotted and understood to be indicative of this possibility, might have led to an intervention that would have prevented this."
Mr. Carter said the reviews would also look at the contractors tasked with checking personnel. But he said that the question of whether the responsibility for conducting security investigations should be returned to the government would be considered by the administration's governmentwide investigation. USIS, the company that conducted background checks of both Mr. Alexis and Edward J. Snowden, the former government contractor who leaked national security secrets, is under investigation by the Office of Personnel Management.
Paul N. Stockton, a former assistant secretary of defense for homeland defense and Americas' security affairs, and Adm. Eric T. Olson, a former head of the military's Special Operations Command who retired from the Navy, will lead the panel that will review security and emergency response at military installations, as well as examine possible problems in the background check process, Mr. Carter said. Mr. Stockton led the internal review of the 2009 shootings at Fort Hood.
A comprehensive report incorporating the findings of all three reviews is scheduled to be delivered to Defense Secretary Chuck Hagel by Dec. 20.
On Wednesday, Hewlett-Packard terminated its working relationship with the computer services company that employed Mr. Alexis for about a year, The Experts Inc. Mr. Alexis worked at numerous military installations for The Experts, including the Navy Yard, where he started work just a week before the shootings.
Hewlett-Packard "has lost all confidence in The Experts' ability to meet its contractual obligations and serve as an H.P. subcontractor," said Hewlett-Packard's director of global contingent labor, Henry Dreschler, in a letter to The Experts' chief executive, Thomas E. Hoshko.
The letter, a copy of which was obtained by The New York Times, added, "It is clear The Experts failed to meet H.P.'s expectations of its subcontractors and suppliers."
A Hewlett-Packard spokesman, Michael Thacker, declined to comment on the letter. "Based on what we now know about The Experts' conduct," he said in an e-mail, "including its failure to respond appropriately to Aaron Alexis's mental health issues and certain incidents recently reported in the press, H.P. has terminated its relationship with The Experts."
Mr. Hoshko did not returns calls for comment.
Serge F. Kovaleski contributed reporting from New York.
