Twelve people were killed and multiple people were injured after a mass shooting at a Southern California bar late Wednesday.

About 40 miles west of Los Angeles, a gunman opened fire inside a country-western dance bar in Thousand Oaks. At least 11 victims were killed at the scene; a deputy later died from his injuries, Ventura County Sheriff Geoff Dean said. The gunman, later identified by police as Ian David Long, was found dead inside.

The bar was hosting an event for college students at the time.

"There is blood everywhere," Dean said Thursday morning.

This is the deadliest mass shooting in the United States since 17 students and teachers were killed at a Parkland, Florida, school in February.

Here's what we know:

What happened?

A gunman dressed in black, later identified by police as Long, 28, began firing at Borderline Bar and Grill around 11:20 p.m. Wednesday, authorities said.

Sheriff Sgt. Ron Helus was among the first to arrive on the scene, entering the bar with a California Highway Patrol officer. Helus exchanged gunfire with the suspect before being shot multiple times. He later died at a hospital.

After spraying bullets for almost three minutes and killing 11 other victims, the gunman then went into an office near the front of the bar where he was found dead, apparently killing himself, police said.

More:Police officer, 11 other victims dead in Thousand Oaks, Calif., bar shooting

More:People threw barstools through window to escape Thousand Oaks, California bar during shooting

Who was the shooter?

Authorities identified the shooter as Ian David Long, a 28-year-old Marine veteran. He used a Glock 21 – a brand of .45-caliber handgun – with an extended magazine, police said. His motive and death are under investigation.

The sale of the high-capacity magazine used is prohibited under California law and possessing it would have been illegal, too, by a state law currently blocked by a federal court.

More:Extended magazine used in California shooting can't be bought legally there

Long, from Newbury Park, California, was known to law enforcement for minor offenses, including traffic violations.

In April, Dean said his department was called to Long's home because of a domestic dispute, where he was "somewhat irate and acting irrationally." A mental health crisis team concluded that Long didn't need to be taken into custody at the time.

Neighbor Richard Berge said everyone on the block was aware when the police came to the Long home earlier this year after an incident.

"Police were here, and they finally get (Long) to come out of the house," Berge told USA TODAY Thursday as FBI agents shuttled in and out of the home Long shared with his mother, Colleen.

More:What we know about the Thousand Oaks gunman Ian David Long, a military veteran

A Marine corporal, Long served for five years from 2008 to 2013, including a tour in Afghanistan, according to Marine Corps records.

Who were the victims?

Authorities still haven't named all the victims in the attack.

Telemachus Orfanos — a survivor of last year's massacre at a Las Vegas country music festival, where 58 people were slain — was killed Borderline Bar and Grill Wednesday.

"My son was in Las Vegas with a lot of his friends, and he came home. He didn't come home last night," his mother, Susan Schmidt-Orfanos, told a local TV reporter. "I don't want prayers. I don't want thoughts. I want gun control."

Daniel Manrique, 33, was a Marine Corps veteran who started the Ventura County chapter of Team Red, White and Blue in 2014 to help fellow veterans make the transition from military to civilian life, says friend Sara Bergeron. She suspects Manrique struggled with PTSD himself, though he never discussed it with her.

When she learned that the Borderline gunman had been in the Marines and may have suffered from PTSD, she thought immediately: "Wow, I wish this person had met Dan."

Noel Sparks, 21, was a college student and a regular at Borderline, where she spent Halloween and celebrated her 21st birthday in August. She often went there with friends and her mom. She was majoring in art at nearby Moorpark College.

Alaina Housley, 18, was a Pepperdine University freshman killed in the shooting. Her aunt was Tamera Mowry-Housley, "The Real" host and "Sister, Sister" star.

Jason Coffman received confirmation that his son Cody, 22, who was about to join the Army, was dead. Coffman broke down as he told reporters how his last words to his son were not to drink and drive and that he loved him.

More:Young people with 'so much life ahead' of them: These are the Thousand Oaks victims

"I cannot believe it's happened in my family," Coffman, 41, said. "I am speechless and heartbroken. ... My life has changed now forever."

Justin Meek, 23, who graduated from California Lutheran, was working Wednesday night as a security guard at the Borderline Bar and Grill when the shooting started. He died trying to save others, according to the university and several social media postings.

Helus 'died a hero'

Sheriff’s Sgt. Ron Helus, who arrived first on scene, died a hero," Dean said. "He went in to save lives, to save other people."

Helus, a 29-year veteran of the department, was looking to retire "in the next year or so," Dean said. The sheriff also described Helus as hardworking and dedicated sheriff’s sergeant.

Helus leaves behind a wife and son.

Before heading to the scene, Helus talked to his wife, as he does several times during his shift, Dean said.

"Hey I gotta go handle a call. I love you. I’ll talk to you later," Dean said of the conversation.

More:Ventura County Sheriff Sgt. Ron Helus among those killed in bar shooting

A popular college bar

The shooting took place inside a popular college bar, Borderline Bar and Grill in Thousand Oaks, known as a safe community to residents.

The bar advertises itself as Ventura County's largest country dance hall and live music venue with a dance floor measuring over 2,500 square feet. On Wednesday nights, including the night of the shooting, the bar hosts a college country night, according to its website.

“Multiple” Pepperdine University students who attend school in Malibu were at the bar during the shooting, the university said Thursday morning.

“At this time, the University is working to identify and provide support to those students,” the university tweeted.

How victims and their families can find help

Ventura County has set up a Family Assistance and Reunification Center at the Thousand Oaks Teen Center, 1375 E. Janss Road.

The American Red Cross and chaplains are at the center to assist.

For family and friends looking for information, an emergency hotline is available at 805-465-6650.

More:Where Thousand Oaks shooting victims and their families can find help

How many shootings have there been this year?

Thousand Oaks is now the site of the 307th mass shooting in the U.S. this year.

And an even grimmer statistic was marked: The 307th mass shooting took place on the 311th day of the year – an average of a deadly incident almost every day so far this year.

In all, 328 people died in mass shootings incidents, and 1,251 injured, according to data from the Gun Violence Archive, a not-for-profit organization that provides online public access to information about gun-related violence. The numbers include incidents in which four or more people were shot or killed, not including the shooters, according to the archive.

More:Thousand Oaks makes 307 mass shootings in 311 days

Contributing: USA TODAY staff, Ventura County Star, The Associated Press.