At least 12 people have been killed, including a police officer, at a bar in Thousand Oaks, California.
Police say the shooting began at 23:20 local time on Wednesday at the Borderline Bar and Grill about 40 miles (65km) north-west of Los Angeles.
At least another 10 were injured in the attack.
Officials say the suspect was found dead inside and have not yet identified him. They do not yet know his motive for the shooting.
At least 200 people were reportedly inside the bar attending a college country music night at the time.
“When deputies arrived they heard shots being fired, it was an active shooter situation,” Ventura County Sheriff’s office spokesman Eric Buschow said earlier.
The police officer who was killed has been named as Ventura Sheriff’s Sergeant Ron Helus – a 29-year force veteran and father-of-one who was due to retire next year.
He was shot several times and later declared dead at a local hospital.
Dozens of shots were reportedly fired by the suspect – who police believe may have taken his own life.
People inside the bar had to break windows to escape, while others sheltered inside toilets.
Reports say the suspect may have used smoke grenades and at least one weapon, described as a semi-automatic handgun by witnesses, during the attack.
A man who was injured told local television channel KTLA that the shots sounded “like fireworks”.
“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,” he said.
Another witness from inside, Teylor Whittler, described the panic inside the venue.
“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!” she said.
“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. BBC