In surveillance footage newly released by the FBI, Aaron Alexis, the 34-year-old gunman allegedly responsible for killing 12 people at the Navy Yard in Washington, D.C., can be seen entering the complex with a case and a backpack, before walking through the halls with a fully assembled sawed-off shotgun.

The FBI also released photos from the scene of the shooting, including one detailing the carvings on his shotgun — which included the phrase "End the torment" — and another showing his backpack hanging in the bathroom stall where he reportedly assembled his gun.

An investigation also found that Alexis acted alone and not, as was previously speculated, with the help of a second shooter or accomplice. The FBI also found "multiple indicators that Alexis held a delusional belief that he was being controlled or influenced by extremely low frequency (ELF) electromagnetic waves," which would explain another phrase carved on his shotgun: "My ELF weapon!"

After a shoot-out with police, investigators found the shotgun and a handgun, believed to have been taken from one of his victims, near Alexis' body.

The Remington 870 shotgun with a sawed-off barrel and stock. (FBI)

"End the tormet!" is scratched into the barrel of the gun (FBI)