The FBI has released chilling footage of gunman Aaron Alexis during the Washington Navy Yard as they updated their investigation into the incident.

The footage shows Alexis arriving at Building #197 where he killed 12 people in a blue Toyota Prius. The closed-circuit video then shows Alexis walking around the building, Remington 870 shotgun in hand. He later got a Beretta handgun while on his rampage. Investigation to date has determined that Alexis acted alone.

The following timeline details Alexis’ activity during the morning of September 16, 2013 according to the FBI's investigation.

  • 7:53 a.m. - Alexis’ rental car, a blue Toyota Prius with New York plates, entered Parking Garage #28 at the Washington Navy Yard, located directly across from Building #197.
  • 8:08 a.m. - Alexis exited Parking Garage #28 on foot carrying a backpack. Alexis entered Building #197 and proceeded to the elevator.
  • 8:09 a.m. - Alexis exited the elevator on the fourth floor and entered the men’s bathroom carrying a backpack and a clipboard.
  • 8:15 a.m. - Alexis crossed the hallway into the 4 West area of Building #197 with shotgun, but without the backpack or a clipboard.
  • 8:16 a.m. - Alexis shot the first victim in the 4 West area of Building #197.
  • 8:17 a.m. - First 911 call is received from the fourth floor of Building #197.
  • 8:20 a.m. - Alexis left the fourth floor using the stairs and entered the third floor.
  • 8:28 a.m. - Alexis appeared on the first floor.
  • 8:57 a.m. - Alexis returned to the third floor.
  • 9:25 a.m. - Law enforcement shoot and kill Alexis on the third floor.

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.

The Associated Press contributed to this report