Twelve people including a police officer were killed when a man opened fire in a crowded country and western bar in Thousand Oaks, California on Wednesday night.
The slain Ventura sheriff's deputy has been identified as Sergeant Ron Helus - a 29-year force veteran and father-of-one who was due to retire next year.
At least 200 people were reportedly inside the bar attending a college country music night at the time.
The gunman has been identified as 28 year-old Ian David Long, a Marine Corps veteran.
Police officers found him dead inside the bar.
Twenty-one people hurt in the shooting have been treated and released from hospitals.
Ventura County Sheriff Geoff Dean say Long used a .45 caliber Glock pistol during the shooting.
The US Marine Corps veteran is suspected to have had mental health issues. Earlier this year, police mental health professionals cleared him after he was found behaving "irrationally" at his home.