During a visit to the resort town of Newport, Rhode Island in August, just weeks before the rampage, Alexis complained to police that people were talking to him through the walls and ceiling of his hotel room and sending vibrations through his body to keep him from sleeping.
The 34-year old former Navy reserve sailor was shot and killed during a gun battle with police 30 minutes after he opened fire on employees of the Naval Sea Systems Command.
The FBI says he acted alone and got into the Navy Yard with valid identification. His motive is still unknown.
Alexis had been previously arrested in two separate shooting incidents in 2004 and 2010, and was cited several times for misconduct during his time in the Navy.