The FBI has released chilling video and photos of gunman Aaron Alexis entering Building 197 at the Naval Sea Systems Command headquarters last week and stalking victims with a shotgun.

The FBI released a report Wednesday detailing its extensive findings into the 34-year-old, who they say acted alone in the shootings that left 12 people dead and four others injured dead before he was killed by police.

We now have a “greater understanding of recent movements and activities” by Alexis, said Valerie Parlave, Assistant Director in Charge of the FBI’s Washington Field Office.

Alexis purchased a Remington 870 shotgun and ammunition at a gun shop in Northern Virginia two days before the shooting. On the same day, he also purchased a hacksaw, among other items, at a home improvement store in Northern Virginia.

Alexis sawed off the barrel and stock and etched the phrases “End to the torment,” “Not what y’all say,” “Better off this way” and “My ELF weapon.”

He also used a pistol he took from a security guard during the shooting spree.

Parlave said there are “multiple indicators” that Alexis held a delusional belief that he was being controlled by extremely low frequency electromagnetic waves.

“Ultra low frequency attack is what I’ve been subject to for the last 3 months, and to be perfectly honest that is what has driven me to this,” he wrote in electronic media found on his computer.

The FBI also released a detailed timeline of Alexis’ activity during the morning of September 16, 2013:

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.

The Associated Press contributed to this report.