Washington Navy Yard gunman Aaron Alexis left a note saying he was driven to kill by months of bombardment with extremely low-frequency radio waves, the FBI said.

The former Navy reservist who slaughtered 12 people at the Washington Navy Yard had been “hearing voices” and was being treated for mental problems in the weeks before the shooting rampage, but was not stripped of his security clearance, officials said.

The Washington massacre gunman used a valid security pass to get inside the navy complex where he killed 12 people but the motive for the shootings was still a mystery, investigators said.

Aaron Alexis seems a study in contradictions: a former Navy reservist, a Defence Department contractor, a convert to Buddhism who was taking an online course in aeronautics. But he also had a temper that led to run-ins with police over shootings.

The defence contractor who fatally shot 12 people at the Washington Navy Yard was carrying three weapons during the rampage.