Ventura County Sheriff Geoff Dean says Long is believed to have killed himself late Wednesday at the Borderline Bar and Grill in the city of Thousand Oaks.
Dean said the Long was armed with a Glock 21, a .45-caliber handgun designed to hold 10 rounds plus one in the chamber.
But the sheriff says the gun had an extended magazine that is illegal in California. The sheriff says he does not know how many rounds that magazine can hold.
Dean says his department had several previous contacts with the former Marine including a call to his home in April, when deputies found him acting irate and irrationally.
The sheriff says a mental health crisis team was called at that time and concluded that Long did not need to be taken into custody.
Dean says the other prior encounters were a traffic accident and an incident when he was the victim of a battery at a bar.
Long was found dead in the Borderline Bar and Grill after 12 people were shot to death late Wednesday.
One of those killed was sheriff’s Sgt. Ron Helus, who was cut down when he tried to enter the club with Highway Patrol officers responding to the gunfire. The others killed have not been identified.
One other person suffered a gunshot wound and as many as 15 others suffered minor injuries from jumping out windows and diving under tables.