The FBI said there was no indication that the Washington navy yard gunman targeted any specific individuals when he opened fire inside the complex, killing 12 people.
The agency released pictures and video of Aaron Alexis stalking the corridors of the building, and held a news conference to give more details of the investigation.
Valerie Parlave, head of the FBI's field office in Washington, said investigators were continuing to explore the background and motivations of 34-year-old Alexis. She said Alexis had a well-documented history of mental health issues.
Parlave said 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.