No fewer than 12 persons have been killed, including a police officer, at a bar in Thousand Oaks, California.
The shooting was carried out about 40 miles (65km) north-west of Los Angeles, police said.
At least 200 people were reportedly inside the Borderline Bar and Grill at the time of the attack.
The suspect was later identified on Thursday as 28-year-old Ian David Long who had served in the Marine Corps. Officials said he was found dead inside.
A college country music night was under way when the suspect, apparently dressed in black, entered the bar and opened fire.
Reports said David may have used smoke grenades and at least one weapon, described as a semi-automatic handgun by witnesses.
An injured man told a local media that, “We just dropped to the ground; we heard a lot of screaming. My friend is the DJ so she cut off the music; we just heard a lot of mayhem.”
Another witness from inside, Teylor Whittler, said: “I was on the dance floor and I heard the gunshots, so I looked back and then all of a sudden everyone screamed: Get Down!”
“It was a huge panic, everyone got up, I was trampled, I was kind of left on the floor until some guy came behind me and grabbed me and dragged me out.”
People reportedly escaped the bar by using chairs to break windows, while others reportedly sheltered inside the venue’s toilets.
Ventura County Sherrif Geoff Dean described the scene inside the bar as “horrific” and said there was “blood everywhere.”
He said first responders had arrived just three minutes after the first emergency calls from the venue were made.
Ventura Sheriff’s Sergeant Ron Helus, who later died in hospital, had tried to enter the active scene with a local highway patrolman when he was shot several times.
He is described as a 29-year officer and a father-of-one, who was due to retire next year.
“It saddens us all and tears at our emotions. He died a hero. He went in to save lives, to save other people,” the Sherriff said.
Police said they recovered a handgun inside the bar and found the suspect dead inside.
The other 11 victims inside the bar were not identified as at the time of this report but the Wednesday night country music event is popular with local university students.
Footage on local media of the aftermath showed injured people being carried away from the scene by their friends.
The Ventura County sheriff, Geoff Dean, said that the suspect, of Newbury Park, Calif., apparently took his own life after being confronted by officers responding to the Wednesday night attack. The .45-caliber handgun he used was purchased legally, but had been outfitted with an extended magazine.
The shooting came just over a year after 58 people were killed at a country music festival in Las Vegas when a gunman opened fire from a high-rise hotel room. There was an eerie parallel between the two shootings as some of the same people who emerged from the bar, the Borderline Bar & Grill, described having survived the shooting on the Las Vegas Strip.
Deputies had some interaction with Mr. Long the last few years, the sheriff said, including a reported disturbance at his home in April that prompted mental health specialists to talk to him. The health specialists, who discussed with Mr. Long his military service and whether he had suffered from post-traumatic stress disorder, determined that he was not an immediate danger to himself or others and that he could not be involuntarily taken to a mental hospital.
Witnesses recalled a chaotic scene at the bar, which was filled with hundreds of people, many of them college students: A gunman opening fire, first at a security guard, as patrons dropped to the dance floor, hid under tables and broke windows to escape.
Sheriff Dean said that at least six off-duty officers were inside when the gunman opened fire. A parent told Sheriff Dean that “they stood in front of my daughter” and protected her. Witnesses also told local news media about patrons in the back of the bar who broke out a window and helped a number of people escape.
The number of people wounded in the shooting on Wednesday was unclear, but the Ventura County Sheriff’s Office said about 22 people had been taken to various hospitals while others are self-reporting with injuries at local hospitals..
The sheriff, his voice cracking, identified one victim as Sgt. Ron Helus, who was shot when he entered the building.
“He died a hero because he went, he went in to save lives, to save other people’s lives,” Sheriff Dean said.
Country music was playing in the dimly lit bar when people first heard gunshots some time before midnight. Some said they had initially mistaken the sounds for firecrackers.
Sheriff Dean said Mr. Long first shot a security officer outside the club and then walked inside, turned to the right and shot “other security and employees” before starting to fire on patrons in the main part of the club.
“No idea what the motive was,” the sheriff said. “We don’t believe he was targeting people.”
“I just started hearing these big pops,” said a witness interviewed by a local television station. The gunman also used smoke bombs, said the man, who was not identified, a detail that the sheriff could not confirm.
Panic ensued as people tried to flee.
“He just kept firing,” the witness said, adding that “people were trying to get out the window” to run away from the gunman, who had “a big handgun.”
A junior at California State University Channel Islands, Chyann Worrell, said she was at the bar to celebrate the 21st birthday of her friend Nellie Wong for a night of line-dancing with a live D.J. Shortly after 11 p.m., Ms. Worrell said, the gunman, wearing dark clothing and a dark baseball cap, drew his gun. He aimed it at a man near the front of the bar.
Ms. Worrell ducked for cover and heard a barrage of bullets. As she ran out of the bar, she said, she saw several bodies sprawled on the floor. Hours after the shooting, she had still not heard from two friends who had been with her at the bar.
One young woman inside the bar, Teylor Whittler, said the gunman appeared focused and did not appear to be targeting anyone in particular.
“I saw him shoot,” Ms. Whittler said, adding that someone had yelled, “Everybody get down.”
She said she saw him quickly reload his gun and fire again. “He knew what he was doing,” she said. “He had perfect form.”
“People started running to the back door,” she said, and she heard someone shout, “Get out – he’s coming.” She then fled and heard another burst of gunfire.