Transcript for What we know about California bar shooting that left 12 dead
I just started hearing these big pops. Pop, pop, pop. The shots kept going and going and going. We just heard people saying, run, and we booked it as fast as we could. It was sheer panic. Everyone ran and dropped as fast as they could. We got multiple people down. We need a lot of ambulances. And the gunman was throwing smoke grenades. Reporter: The shots rang out during college country night at the borderline bar and grill in Thousand Oaks, California. This video capturing the moment. Watch as the camera shakes. That piercing sound? It's gunfire. Then, silence. For six endless seconds. Now, listen. They run for their lives. Guys, run, he's coming out this door! Reporter: Countless heroes inside the bar helping to create escape routes, like Matt, who helped rescue scores of people by smashing a window with a bar stool. We were just pushing people through, as many as we could, kind of quickly checking back to make sure that he wasn't coming towards us. Reporter: 12 killed, including a police sergeant. He died a hero because he went -- he went in to save lives. Reporter: Jason Coffman, his son, Cody, one of the victims. Last thing I said was, son, I love you. That was the last thing I said. Reporter: The shooter, now identified as 28-year-old Ian David long, a war veteran. Authorities are now trying to determine why and how this could happen in a community that just this year was named one of the safest in America. It was just before 11:30 P.M. And more than 100 people gathered to enjoy live country music. Look at this past video, posted on the bar's YouTube page, illustrating what country night is normally like. Anyone 18 or older is invited to attend. But instead of country music, gunshots. The gunman, dressed in all black, and was armed with a block .45 caliber pistol with an extended magazine. Witnesses say he threw some kind of smoke bomb and began firing into the crowd. He shot the doorman first. And then he turned to the young cashier and he shot her. In a split second, everyone yelled, get down, and everyone pretty much dog piled on top of each other. Reporter: Amid the chaos, the voice of a young man calling home. Mom, dad, this is Matt. There's been a shooting at borderline. Reporter: Sheriff's deputies arrived within minutes. And engaged the suspect in a fire fight. Sergeant Ron helus, a 29-year veteran of the force, was the first one in. Patrons rushing out of the bar, trying to carry friends and strangers to first responders. We just tried to get out, get behind a car, get behind something. We just tried to take cover. By the grace of god, I was able to get to the front door. I saw him at the front door. I'm lucky to be alive. Reporter: Lucky to be alive, a sentiment once again shared by far too many young people. I'm too scared. Reporter: You're too scared? I may not go to school tomorrow. Reporter: Her night was all about celebrating. It was her 21st birthday. As mayhem ensued around her, Nelly found herself hiding behind a bar stool and in that moment, she joined another club, a terrible one, and no longer rare. Those who have survived a mass shooting. Reporter: What were you thinking while you were hiding? What is going on? Why tonight? Why now? Reporter: The alleged shooter's motive, unknown. We believe he shot himself. When the officers went in and made reentry, they found him already deceased. Reporter: Long, a marine combat veteran, served for five years. His mother's Facebook page proudly shows photos of him in uniform. He served a tour in Afghanistan as a machine gunner. Long's Facebook photos paint a picture of a smiling young man. Another photo shows him at a music festival. But his neighbors describe a very different person who was often heard arguing with his mother. They used to argue back and forth quite loud. Reporter: Neighbor Donald mclloyd lives next door to long. One night, about 8:00, 9:00 at night, we hear the -- sounded like gunshot, and I told my wife, told -- told put the light on. Just stay down. Reporter: Today, the FBI was at long's house, removing evidence. Neighbor Richard burg said he tried to talk to long's mother about getting her son help. I said, can't you get help? She said, well, he doesn't want to get help. Reporter: Police say they've had several run-ins with long. They even considered him for a 5150 which allows authorities to involuntarily hold someone they feel is a danger to themselves or others. In April of this year, deputies were called to his house. He was somewhat irate and acting a little irrationally. They called out our crisis intervention team, our mental health specialists, who met with him, talked to him and cleared him, didn't feel that he was qualified to be taken under 5150. Reporter: It's unclear if police knew if he had a gun at the time. There are a growing number of examples around the country where law enforcement is able to step in, to assess the risk posed by an individual, and to take steps to prevent an attack. In some cases, it's providing a person access to mental health services. It could also include taking steps to prevent access to firearms. Reporter: But even when authorities do take action against people they view as a danger to themselves or the community, things can still turn deadly. Sheriff's office. Reporter: Last new year's eve, Colorado police tried to take 37-year-old Matthew reel in for a mental health hold. The scene was captured on police body cam footage. We're going to take him. For what? A mental health hold is when the officers have gotten on scene, he was throwing things off his balcony, yelling and screaming. Reporter: Unbeknownst to the officers, he was armed with a rifle. Sheriff's office. Reporter: When they tried to take him in, he opens fire, shooting three officers, killing one. He was eventually killed in a shootout with police. As these scenes continue to play out across America, the sheriff here in Ventura county tries to make sense of yet another tragedy. It's very frustrating for all of us when you can find no rhyme or reason for this completely inexplicable loss of life. Reporter: In honor of the fallen sergeant, hundreds lined the streets to pay their respects. ??? Amazing grace ??? Reporter: Vigils in honor of those lost. We hold the hands of everyone in our community, and we come together to move forward and overcome evil with good. Reporter: Among the other victims, 18-year-old Alaina Housley, a freshman at Pepperdine university. Tonight, her family says Alaina was an incredible young woman with so much life ahead of her. And 23-year-old Justin meek. Tonight, he is credited with heroically saving lives during the tragedy. Blake was 21. Tonight, his girlfriend writing, my heart is hurting more than words can say. I cannot believe you're gone. I'm so grateful for our little infinity. Tyler and Zach lost two of their friends. One of them had escaped death just a year before during the Las Vegas shooting. What do you want people to know about the two friends that you lost? They were some of the best people that I've ever met. Reporter: Really? They would give you the shirt off their back in a heartbeat. Reporter: Jason Coffman showed a picture of his son, Cody, early this morning. At the time, still searching, still praying, and a few hours later, his worst fear confirmed. We did just get the news that he was one of the 11 that were hit and killed last night. Only him and I know how much I love -- how much I miss him. Oh, god. Reporter: For "Nightline," Thousand Oaks, California.
This transcript has been automatically generated and may not be 100% accurate.