The FBI has released surveillance video of the Washington Navy Yard gunman stalking the corridors as he searches for victims.
It shows Aaron Alexis entering the base armed with a shotgun, prowling the corridors and checking if a room is empty.
Thirteen people were shot dead in the September 16 attack including the assailant Aaron Alexis.
In a press conference FBI spokesperson Valerie Parlave said Alexis believed he was being controlled by electromagnetic waves.
“At this point, I can confirm that there are multiple indicators that Alexis held a delusional belief that he was being controlled or influenced by extremely low frequency, or ELF, electromagnetic waves,” Parlave told journalists.
Photos have also been released of the murder weapon – a Remington shotgun. Alexis, who acted alone, does not appear to have targeted specific people.
The shooting spree has raised questions about how Alexis, who was a government technology contractor, was able to get security clearance to enter the base despite a history of mental health issues and gun misuse.