The chilling video shows Alexis walking through the halls of the Navy Yard building brandishing a shotgun, preparing to attack.
In releasing the video, the agency said there is no indication that the Navy Yard shooter targeted any specific individuals when he opened fire inside a building, killing 12 people.
Valerie Parlave, head of the FBI's field office in Washington, said Wednesday during a news conference that investigators are continuing to explore the background and motivations of 34-year-old Aaron Alexis. However, she says Alexis had a well-documented history of mental health issues.
Parlave says Alexis held a delusional belief that he was being controlled by extremely low frequency waves, or ELF waves. Alexis had written "my ELF weapon" on a shotgun he used in the rampage.
Alexis, a former Navy reservist and IT contractor, was killed in a shootout with police.
Authorities say Alexis had only recently started his job, and that although there was a "routine performance-related issue addressed to him" on the Friday before the shooting, there's no indication that he targeted particular co-workers or was motivated by problems in the workplace, said Parlave.
At the Pentagon on Wednesday, Deputy Secretary Ash Carter said the department will complete three separate reviews in late December, including internal and independent assessments of base safety procedures as well as the security clearance process.
"Bottom line is, we need to know how an employee was able to bring a weapon and ammunition onto a DoD installation, and how warning flags were either missed, ignored or not addressed in a timely manner," Carter said during a Pentagon briefing.
Carter said the reviews will include consideration of Navy Secretary Ray Mabus' recommendation that the department require that all police reports, not just arrests or convictions, involving an individual be included in background checks.
The Defense Department's internal review will be coordinated by the Pentagon's top intelligence official, Under Secretary Michael Vickers, and the independent review will be spearheaded by retired Navy Adm. Eric Olson and Paul Stockton, the former assistant secretary for homeland defense. A Navy review will be finished by the end of October; initial findings of the two larger reviews are due to the defense secretary by Nov. 15; and the final overall report will be done by Dec. 20.