Twelve people were killed in an overnight shooting at a popular bar in a Los Angeles suburb, authorities said.

The victims included a sheriff's sergeant.

Officials also said they believe the suspected shooter took his own life. He was identified as 28-year-old Ian David Long, a former Marine who had multiple contacts with law enforcement, according to The Associated Press.

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The gunman, dressed in black, opened fire inside the crowded Borderline Bar and Grill in Thousand Oaks, Calif., a favorite of college students, the Los Angeles Times reported.

"We have no idea if there is a terrorism link or not," Ventura County Sheriff Geoff Dean told reporters early Thursday.

An undetermined number of people were also injured, Dean said.

He called the mass shooting a "horrific incident."

“This community in my 41 years had never experienced anything like this," he added.

Dean identified the slain sergeant as Ron Helus, 29-year law enforcement veteran.

"Ron was a hardworking dedicated sheriff's sergeant. 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," he said.

A witness told ABC7 in Los Angeles that the gunman walked up to the bar's entrance, shot a security guard and a cashier, and threw a smoke bomb.

“I just started hearing these big pops,” said the witness, John Hedge, of Moorpark, Calif. “The gunman was throwing smoke grenades all over the place.”

Authorities told reporters, however, that they have yet to find evidence of a smoke bomb.

"We have called in our bomb team to go through and a couple of bomb dogs here that are checking the area," Dean said. "But we haven’t found any other explosive device or confirmation there that there was a smoke device.”

President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE said in an early morning tweet that he had been "fully briefed" on the "terrible" shooting and said in a subsequent post that police showed "great bravery."

"God bless all of the victims and families of the victims," he said.

I have been fully briefed on the terrible shooting in California. Law Enforcement and First Responders, together with the FBI, are on scene. 13 people, at this time, have been reported dead. Likewise, the shooter is dead, along with the first police officer to enter the bar.... — Donald J. Trump (@realDonaldTrump) November 8, 2018

....Great bravery shown by police. California Highway Patrol was on scene within 3 minutes, with first officer to enter shot numerous times. That Sheriff’s Sergeant died in the hospital. God bless all of the victims and families of the victims. Thank you to Law Enforcement. — Donald J. Trump (@realDonaldTrump) November 8, 2018

Chris Mills Rodrigo contributed to this breaking news report, which was last updated at 10:26 a.m.