A former Marine opened fire in a crowded bar popular with college students in the U.S. state of California, killing 12 people including a sheriff's deputy, police said on Thursday, in the latest mass shooting that shocked the country.

Local authorities identified 28-year-old Ian David Long as the gunman of the mass shooting Wednesday night in Thousand Oaks, a suburb about 64 km northwest of Los Angeles, which also left 25 people injured.

Ventura County Sheriff Geoff Dean said Long, a Marine Corps veteran, apparently fired at random with a .45-caliber Glock handgun with an extra-capacity magazine inside the Borderline Bar and Grill at about 11:30 p.m. local time on Wednesday.

"The weapon was designed in California to hold a magazine of 10 rounds and one in the chamber, but he had an extended magazine on it," Dean said.

The motive of the shooter was so far unclear, he told a news conference on Thursday morning.

Dean described Long as "acting a little irrationally" when police were called to his home earlier this year to investigate a disturbance. He lived in Newbury Park, near Thousand Oaks.

Garo Kuredjia, captain of Ventura County Sheriff office, told Xinhua that deputies responded shortly after they received calls about the shooting at the bar. The first responding deputy was Sergent Ron Helus, who was killed in confrontation with the shooter.

"When he got to the scene, he saw two California Highway Patrol (CHP) officers outside with a victim that came out of the bar," Kuredjia said.

Helus and CHP officers heard gunshots after they arrived, and entered the bar in order to neutralize that threat, Kuredjia said.

When Helus went in, he was unfortunately shot for multiple times. He was later transported to a local hospital. Unfortunately there are other 11 victims who were also shot dead, Kuredjia said.

The shooter died of self-inflicted gunshot wound, he said.

In the neighborhood where Long lived, residents said they were well aware of his problems. Richard Berge, 77, was quoted by local media as saying the former Marine had post-traumatic stress disorder (PTSD) and was known to kick in the walls of the home.

Kuredjia told Xinhua he could not confirm the shooter's PTSD, and did not know what his medical or mental status was.

"I can assume he has some sort of mental issue, because no normal person would commit such an outrageous act and murder innocent people," Kuredjia said.

A total of 25 other people were injured in the shooting, including 23 who self-transported themselves to hospitals, and two who were taken by ambulance, according to the Ventura County Fire Department.

A procession was held Thursday morning for Deputy Sgt. Helus, a 29-year veteran who was set to retire next year. He is survived by his wife and son.

His body was transported via motorcade from Los Robles Hospital to the Ventura County Medical Examiner's Office. People stood along the route with grief, paying salute to the officer.

Ralph Chapman, whose son is a colleague of Helus, told Xinhua that his son was among the officers running into the bar to respond to the shooting.

Chapman said he was very sad and worried when he heard that one officer was killed at the site, very afraid that it might be his own son.

Chapman's son was a member of the motorcade transporting Helus' body on Thursday morning.

Chapman said he has served at U.S. troops in Vietnam for two years, and that he believed PTSD should not account for the mass shooting.

Tami Teece, who was a survivor of the mass shooting in Las Vegas last year, which killed 59 people and wounded more than 500, was among the crowd mourning the death of Helus along the road.

She, who also has PTSD, told Xinhua that the government should spend more money setting up mental hospitals for veterans to address their psychological problems.

Authorities said hundreds of people were inside the bar when the gunfire rang out. A family assistance center has been set up at 1375 E. Janss Road in Thousand Oaks.

The bar shooting was the latest mass killing in the United States, which with more than 300 million guns scattered nationwide, has long been plagued by gun violence. Less than two weeks ago, 11 worshippers were killed by a gunman at a synagogue in Pittsburgh, the U.S. state of Pennsylvania.