Navy Yard shooting video footage released by FBI shows the horrific attack by Aaron Alexis on the Washington Navy Yard that killed 12 people.

According to the FBI, the video footage from closed-circuit video cameras shows Aaron Alexis at the Washington Navy Yard on September 16, 2013.

Alexis is first seen driving his rental car, a blue Toyota Prius, through the Washington Navy Yard main gate.

Alexis next can be seen entering Building #197 carrying a backpack. Alexis had legitimate access to the Washington Navy Yard as a result of his work as a contractor and he utilized a valid pass to gain entry to the building.

Alexis moves through the hallways and a stairwell of Building #197 carrying a Remington 870 shotgun.

Alexis acted alone, the FBI concluded after an investigation.

The FBI released the following timeline detailing Alexis' activity that day:

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.