(CNN) - A college country night at a bar in Thousand Oaks, California, turned deadly late Wednesday when a gunman forced his way in and opened fire on the young people gathered there. At least 12 people were killed.
Here's what we know:
How the shooting unfolded
The first call of shots fired came in to police Wednesday around 11:20 p.m. PT (2:20 a.m. ET, Thursday), Ventura County Sheriff Geoff Dean and a department spokesman said.
The gunman shot a bouncer or security guard outside and forced his way into the bar, witnesses said. The gunman also shot a young woman working at the counter just inside the door, several witnesses said.
"I see that girl every single time I'm there," witness Holden Harrah told CNN, fighting back tears. "I don't know her name, but (she) is someone I make small contact with and I say hi every time."
Officers arrived on the scene 2 to 3 minutes after the initial call to law enforcement as shots continued to ring out, Dean said.
A few minutes later, Ventura County Sheriff's Sgt. Ron Helus and a California Highway Patrol officer headed into the bar after hearing more shots, Dean said.
As they entered through the front door, Helus was struck by bullets, Dean said. The highway patrol officer stepped back, pulled Helus from the line of fire and secured his post until more officers arrived.
Police identified the gunman as Ian David Long, a 28-year-old US Marine Corps veteran.
He was dead when officers entered the bar, Dean told reporters. He is believed to have killed himself.
The shooter wore a black trench coat and eyeglasses and had a black beard, witnesses said.
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The shooter used a .45-caliber Glock handgun in the massacre, Dean said. The gun was purchased legally, he added, but had an extended magazine on it.
Asked by a reporter if the extended magazine was illegal in California, Dean said, "Yes."
California law bans the manufacture, importation, sale or transfer of large-capacity firearms magazines, but possession of such magazines is not covered. A measure that would have made possession of a large-capacity magazines illegal was challenged in a lawsuit by gun-rights advocates, and a federal judge last year blocked enforcement of that part of the law while the suit is ongoing.
Long was active duty Marine from August 2008 to March 2013, Department of Defense records show.
Law enforcement had minor run-ins with Long over the years, Dean told reporters. In April, deputies were called to his home for an unspecified disturbance.
He was acting "irrationally," Dean said, but a mental health specialist who met with him decided he did not need to be detained under laws pertaining to his mental health.
Long attended Cal State University Northridge between 2013 and 2016, according to a school spokeswoman. He majored in athletic training, but did not graduate.
Police are still trying to determine a motive in the shooting.
At least 12 people were killed, including Helus, a 29-year law enforcement veteran.
Helus, who was set to retire within the next year, succumbed to his wounds at a hospital, police said. He is survived by his wife and a son.
Authorities are still working to identify all the victims and notify their families, Dean told reporters.
The victims' ages weren't immediately known, though it's estimated most were between 21 and 26 years old because "this is a nightclub," he said.
Several other people were wounded or sustained minor injuries trying to escape the shooting, Dean said.
Twenty-one victims were treated at local hospitals and had been released by Thursday morning, according to the sheriff's office.
It was not immediately clear whether those victims had been shot.
Ten patients were treated and released from Los Robles Medical Center, the hospital said in a statement.
Another eight patients were treated and released for minor injuries at Adventist Health Simi Valley, a hospital spokeswoman said. All eight came into the hospital on their own, and none of them were treated for gunshot wounds.
President Donald Trump extended his sympathies Thursday morning on Twitter to the victims and their families, and he praised the "great bravery" shown by law enforcement.
Thousand Oaks is about 40 miles west of downtown Los Angeles and is "considered one of the safest cities in the country," Mayor Andy Fox said.
"The reality is that these types of incidents can happen really at any place, at any time, even in communities that are considered extremely safe," he told CNN.
Borderline Bar & Grill is a western-themed establishment known to regularly host country, salsa and swing dancing nights.
The venue was hosting a college country night from 9 p.m. PT on Wednesday to 2 a.m. on Thursday when the shooting occurred, according to its website. Hundreds of people were estimated to have attended.
The bar is a few miles from California Lutheran University and is a popular spot for students. Some students from California Lutheran and nearby Pepperdine University are believed to have been at Wednesday night's event.
Some freshmen had planned an outing Wednesday to the bar, said Madeleine Carr, news editor at the Pepperdine Graphic, the student newspaper.
"I was going to go with them tonight but decided not to because of homework," she told CNN, "but it is a popular place to go because it's local, and we do have a lot of people from the South -- it's line dancing it attracts them."