Washington Navy Yard gunman Aaron Alexis believed he was being controlled by extremely low frequency waves.
Valerie Parlave, head of the FBI's field office in Washington, said evidence had shown Alexis was acting alone during shooting on September 16, which killed 12 people.
She said investigators were exploring the background and motivations of the 34-year-old, but added Alexis had a well-documented history of mental health issues, Sky News reports.
According to the report, Parlave said Alexis held a 'delusional belief' that he was being controlled by extremely low frequency waves.
Records recovered from Alexis' computer and mobile phone reveal that he had paranoia and mental health problems.
The FBI has released surveillance video and images that show Alexis arriving at the Navy Yard and launching his attack on a mix of military and civilian personnel, the report added.