Ventura County sheriff’s deputies and FBI investigators gather outside the house of Ian David Long in Newbury Park, Calif., on Thursday. (Richard Vogel/AP)

In April, sheriff deputies here were called to a one-story, brown-roofed house on Fowler Avenue in Newbury Park, Calif.

Neighbors on this normally quiet street had heard alarming banging noises and yelling in the middle of the night from within the house. When authorities arrived, they engaged in a standoff with an angry young man inside, at one point drawing their guns and rifles and training them on the house, one neighbor said.

Finally, the man came out and talked to authorities, who ultimately found him not to be a threat. All was quiet again on Fowler Avenue, until Thursday morning when police announced that the same young man — Ian David Long, 28 — had walked into a bar and started firing a Glock handgun into the crowd inside, killing 12.

The massacre inside the Borderline Bar & Grill here was the latest in a series of shootings that has involved a young man exploding in a burst of violence, a pattern that has recurred to devastating effect in Newtown, Conn.; Aurora, Colo.; Charleston, S.C.; Roseburg, Ore.; Parkland, Fla.; and many other places throughout the country.

Authorities found Long dead amid the bloody carnage in the bar after an exchange of gunfire with police. They believe he killed himself.

Long was a military veteran. The Marine Corps said he served between August 2008 and March 2013, including a five-month stint as a machine-gunner in Afghanistan.

His cousin AJ Schramm told CBS News that Long returned a changed man from his deployments and started distancing himself from family. Some relatives, Schramm said, talked among themselves, wondering if Long suffered from PTSD.

An obituary in the Santa Ana Orange County Register said Ian Long’s father, David Bruce Long, died in 1999 of liver failure, when Ian Long was 9 years old.

In 2006, Long and his mother, Colleen Long, moved into the home on Fowler Avenue, according to property records. Neighbors said his mother was friendly, quick to exchange a nice word during her frequent walks with her three German shepherds.

But they described in vivid words the angry and troubled nature of her son.

Carol Richardson, who lives a few houses down from the Longs, said a friend went to the home a few times to “calm him down.” After Wednesday night’s shooting, Richardson said her son texted her, “I bet it was that guy.”

“We always knew he had problems,” said her 19-year-old daughter, Morgan Richardson.

The neighborhood is a quiet one — middle-class homes built in the 1960s, with American flags hanging on porches and work trucks in many driveways.

Little excitement ever happens here, neighbors said, which is why many were alarmed in April during Long’s standoff with police.

Richard Berge, who lives around the corner from the Longs, said he woke up to police cars blocking the street, so he walked to see what was going on.

“I saw the police behind a wall with a rifle and across the street with a rifle,” Berge said.

The retired 77-year-old Vietnam veteran said he spoke with Colleen Long after the incident.

“I told her my own son committed suicide at 25, and she said that’s what she was worried about,” Berge said.

Deputies who responded to the incident said Long was irate and acting irrationally, said Ventura County Sheriff Geoff Dean. The sheriff’s crisis-intervention team and mental health specialist met with Long, and those who evaluated him discussed the possibility that “he might be suffering from PTSD,” Dean said, because of Long’s service.

Under a 5150 order, authorities are allowed to take people into custody and hold them for up to 72 hours because they are considered a threat to themselves or others. But the mental health crisis team that evaluated Long cleared him, Dean said. If they had placed him under a psychiatric hold, he would have lost the legal right to own a gun.

Police have had “several contacts” with Long over the years, Dean said, most of them for minor events including traffic accidents. He said the gun used in the bar massacre appeared to have been purchased legally.

Long’s relatives — including his mother, Colleen — did not return calls for comment.

Long attended Newbury Park High school, where he played baseball, according to team rosters, and later attended California State University at Northridge. The college confirmed Long was a former student who last attended the school in 2016.

Blake Winnett, who lived with Long for a few years, most recently in 2014, said he never saw signs of PTSD or mental health problems in his former roommate.

“He was just quiet . . . did his own thing,” said Winnett, 35. “Kept his door closed all the time.”

Long listened to music constantly, he said, rarely appearing without his ear buds. He recalled seeing Long dancing alone in their garage on several occasions.

“I would open the garage and he’d be in there all sweaty,” Winnett said. “He would put his laptop on the dryer playing this EDM trance music and it’d be like 100 degrees in there. But he would always keep the garage shut.”

Winnett said he was not close to Long, but they went out together a few times including to nightclubs. His mother would visit often and worry over him.

“If he had his truck in the shop, she would drive her car all the way out to the Valley to drop it off for him to use,” he said. “She paid for his rent, I think, because he didn’t have a job. He was going to school.”

Winnett said Long never mentioned he had been married. Court records show he wed a woman named Stavroula in June 2009, after he entered the Marines.

Long’s deployment to Afghanistan lasted from November 2010 to June 2011, a period of intense conflict with the Taliban insurgency as the Obama administration poured U.S. troops into the area and the number of American fatalities and wounded rose sharply. Long was awarded a Combat Action Ribbon, signifying he saw combat.

Long left the Marines as a corporal on March 2013, and just two months later, he and his wife divorced.

A young woman answered the door at his ex-wife’s listed address at an apartment building in Beverly Hills, where a package was waiting addressed to “Vivi.”

When a Washington Post reporter identified herself, the woman slammed the door.

Two FBI agents in plain clothes arrived shortly after.

Through the closed door, one of the agents identified himself.

“You’re not in trouble,” he said. “We just want to ask you a few questions.”

The same woman opened the door and said, “I’m grieving, just like everyone else in the world.”

She told the agents she was “terrified” that her name would be revealed, and then let them inside. The agents left about an hour later.

Long’s last duty assignment was with 2nd Battalion, 3rd Marines, an infantry unit based out of Kaneohe Bay, Hawaii. Aside from the Combat Action Ribbon, his awards include a Navy unit commendation, a Global War on Terrorism service medal and an Afghanistan campaign medal.

A person who identified himself as “Ian” and served in the same places and at the same times as Long posted on ShadowSpear, an online news and community network for special-operations forces. In 2012, he asked for ideas about what he might do after he left the service.

“I have deployed twice, once to Afghan (Marjah) and currently to Japan (Okinawa),” he wrote. “I am getting out soon but I definitely want to become a SOF operator. I am not sure exactly which species.”

Five years later, in March 2017, the person, writing under the username Doorkicker03, returned to the website, drawn there, he said, by the professionalism of special-operations forces members, whose advice he again sought.

“I am graduating with a B.S. in Athletic Training in two months,” he wrote. “I found out a little too late that just wasn’t the job for me. Maybe the ego got the better of me but it took only one time for a 19 year old D-2 athlete to talk down to me and tell me how to do my job that I realized this wasn’t the career I wanted to head.”

He wrote that he hoped to get into the 75th Ranger Regiment — a lofty goal. The regiment is an elite light-infantry unit that has been prominently used in Afghanistan’s eastern mountains in recent years to carry out night raids on both the Taliban and the Islamic State.

“I am choosing to stay enlisted,” he wrote, “because of how much quicker (with my prior service) it would be to get into the regiment rather than going to the dark side.”

Wan and Sellers reported from Washington. Rob Kuznia in Ventura, Calif.; Annie Gowen in Beverly Hills, Calif.; and Mark Berman, Alice Crites, Steve Hendrix, Jennifer Jenkins, Dan Lamothe, Julie Tate and Emily Wax-Thibodeaux in Washington contributed to this report.