The FBI was carefully examining Thursday the vehicle and home of the gunman who killed 12 people at a bar in Thousand Oaks, Calif. They heard gunfire and went inside.
They also were concerned that Long might be suffering from post-traumatic stress disorder, Mr Dean said, "basing that (on) the fact that he was a veteran and had been in the Corps".
Dean identified the suspect as Ian David Long, a Marine Corps veteran and resident of Thousand Oaks, an upscale city northwest of Los Angeles.
His military service lasted almost five years, and he was honourably discharged with the rank of corporal in 2013, the Pentagon said.
Court records show Long was married as a 19-year-old in Honolulu in June 2009, and he and his wife separated in June 2011 while he was deployed to Afghanistan.
Witnesses of the tragedy revealed they only managed to escape in the split-seconds when Long was reloading.
The police captain also did not know whether the gunman had been "neutralised" or not calling the situation "fluid".
Authorities so far had no indication of a motive, or of any connection to terrorism. Later, he enrolled at a university and most recently lived with his mother in a home where neighbours said they could hear aggressive arguments. "Until I find out particularly what the specifics are with this kid who did the shooting I'm not gonna be vilifying him".
Soon after the shooting started, Sergeant Ron Helus, a veteran of 29 years, was first on the scene and entered the bar with another policeman.
The Ventura County Sheriff's Department said 21 people had been treated for injuries at area hospitals and released.
At an earlier news conference Sheriff Dean's voice cracked as he spoke about Sgt. Helus, "He went in to save lives. He was totally committed, he gave his all, and tonight, as I told his wife, he died a hero; he went in to save lives, to save other people".
'I ran out the front door... "It sounded like fireworks or something", said witness John Hedge, according to NBC News.
Paul Delacourt, assistant director in charge of the Los Angeles FBI office, said it was too early to speculate on the shooter's motives but that he appeared to have acted alone. "We're all a big family and unfortunately this family got hit twice".
"My friend killed my other friends, and I was just there to see it all happen", Stratton said.
The Los Angeles Times quoted a law enforcement official as saying at least 30 shots had been fired and that there were casualties.
"We were just at the bar, having fun, dancing", she told reporters.
About 18 months ago, Don and Effie MacLeod heard "an terrible argument" and what he believes was a gunshot from the Longs' property.
Suddenly, a man wearing a black trench coat and glasses approached the bar entrance and shot the security guard.
"It's a horrific scene in there". Long had fatally shot himself inside an office next to the bar's entrance, Dean said. However, mental health specialists who were called in "talked to him, and cleared him".
Authorities said the.45 caliber Glock handgun used in the shooting was legally purchased.
People wait in line to donate blood outside LaReina High School after a mass shooting by a lone gunman at a bar in Thousand Oaks, Calif., Nov. 8, 2018.
Matt Wennerstrom said he instinctively pulled people behind a pool table, and he and friends shielded women with their bodies after hearing the shots.
Telemachus Orfanos survived the deadliest shooting in modern American history. A sheriff's deputy, 12 attendees and the gunmen were killed.
President Donald Trump praised police for their "great bravery" in the attack and ordered flags flown at half-staff in honor of the victims.
Trump ordered the USA flag at the White House and other government buildings to be flown at half-staff.
Among those outside the hospital was Ellen Rivera, who said she had survived the October 2017 slaughter of 58 people at an outdoor concert in Las Vegas - the worst mass shooting in United States history.
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