During a press conference, federal investigators released video and photos of Aaron Alexis entering the Navy Yard's Building 197 at 8:08 a.m., and then waking in a hallway carrying a shotgun.
The shotgun, which had been bought only days before the attack at a Virginia gun store, had been altered, with a sawed-off barrel and stock.
Authorities say Alexis did not target particular individuals during the Sept. 16 attack that left 12 people dead, and the shooting is not believed to have stemmed from any workplace dispute.
The FBI believes Alexis had a "delusional belief" that he was being controlled by electromagnetic waves, and that was what drove him to kill.
"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," read an electronic document agents recovered after the shooting.
Investigators say they believe Alexis expected to be killed when he launched his attack. Officials said he had etched "end to the torment" on the shotgun's barrel.
The attack came one month after Alexis had complained to police in Rhode Island that people were talking to him through the walls and ceilings of his hotel room and sending microwave vibrations into his body to deprive him of sleep.
ABC News and The Associated Press contributed to this report.