- Video report by ITV News correspondent Dan Rivers
A suspected gunman who opened fire at a busy California bar, killing 12 people, has been identified as a former US Marine Corps veteran.
The sheriff in the city of Thousand Oaks named the shooter as 28-year-old Ian David Long, and confirmed he had been found dead inside the bar.
One of Sheriff Geoff Dean's deputies, Sgt Ron Helus, was among those killed, after he responded to emergency calls around 11.20pm local time on Wednesday.
Also among the dead was 22-year-old Cody Coffman, whose father Jason confirmed his son had been killed in the attack.
A number of others were hurt in the shooting at the Borderline Bar & Grill in Thousand Oaks, suffering "different levels" of injuries.
Police said they still had "no idea" whether there was a terrorism link.
Sheriff Dean said the shooting was a "tragic, tragic situation".
He reported that the eleven revellers were found dead inside the bar, and that Sgt Helus suffered fatal injuries when he made his way inside.
"He went into save lives, to save other people," he said.
"He was totally committed, he gave his all, and tonight, as I told his wife, he died a hero."
Donald Trump praised the police in Thousand Oaks, saying that officers who responded to the shooting had shown "bravery".
"I have been fully briefed on the terrible shooting in California. Law Enforcement and First Responders, together with the FBI, are on scene," the president tweeted.
"13 people, at this time, have been reported dead. Likewise, the shooter is dead, along with the first police officer to enter the bar.
"Great bravery shown by police. California Highway Patrol was on scene within 3 minutes, with first officer to enter shot numerous times."
The Borderline bar had been hosting a college country music night.
Footage on social media in the immediate aftermath showed people being carried away from the scene.
Witnesses described the gunman as wearing a ski mask, baseball cap and dressed in all black.
Authorities said they responded to a "shots fired" emergency call, and that some officers on arrivals heard shooting.
Witnesses told ABC News a man fired several shots from a handgun before tossing smoke grenades and firing again.
Sheriff Dean confirmed he had heard reports that smoke grenades had been used.
He also gave some details on the suspect, who was found dead in the bar when police arrived at the scene.
Long was a US Marine Corps veteran, whose Department of Defense records reveal as being on active duty from August 2008 to March 2013.
He had used a handgun, with a Glock .45-calibre weapon found nearby.
It usually holds 10 rounds plus one in the champer, but Sheriff Dean said he had used an extended magazine in this shooting.
Sheriff Dean said authorities had had "minor" run-ins with Long before, including a traffic collision and a call to his home in April, when deputies found him acting in an "irate and irrational" manner.
The sheriff said a mental health crisis team was called at that time, and concluded that Long did not need to be taken into custody.
One witness, a regular at the Borderline Bar & Grill, described hearing "very loud" gunshot sounds inside the venue.
Erika Sigman said the shooting was "very fast and very rapid", adding that she also saw smoke.
Ms Sigman said that she and friends then lay on the ground because many people were screaming
"Once the shots kept going and going and going we just heard people say run and we hooked it as fast as we could," she said.
Another witness, Tayler Whitler, reported: "I was on the dancefloor dancing at the time, all my friends were at the table by the door.
"I heard the gunshot, turned around and saw him shoot a couple more times.
"Within a split second everyone yelled 'get down'."
She added that there was "huge panic" as people attempted escaping, with some using barstools to break windows.
The shooting sparked calls for tighter gun control.
One woman, in tears as she addressed reporters near the scene, said the attack "could have been prevented".
"How many more times is this going to happen?" she said.
"This is going to happen again... it's not right."
It was a sentiment echoed by California Senator Dianne Feinstein, who, in a statement, said a murderer-to-be "barely has to lift a finger" to buy a gun - and criticised the Republican party for refusing to take action on the issue.
"Today we add Thousand Oaks to the ever-growing list of communities that have suffered mass shootings," she said.
"These mass murders are depressingly pervasive. Schools. Theaters. Malls. Offices. Synagogues. Grocery stores. Bars. Concerts. Churches. They're inspired by racism, revenge, terrorism or just pure hatred. The one common attribute: easy access to guns.
"Some will say California's strong gun laws didn't prevent this shooting, but without stronger federal gun regulations, there's little California can do to keep guns coming in from other states.
"Without stronger laws to prevent straw purchasing and close the gaping holes in our background check system, a presumptive murderer barely has to lift a finger to buy a gun.
"Some will say we need more guns, not fewer - an absurd claim in a bar full of students with security guards where an armed police sergeant was killed.
"Republicans argue we shouldn't talk about gun policies after a mass shooting. They're wrong. This is exactly the time we should be talking about enacting gun safety measures. We don't know all the details of this latest shooting, but we do know how to start to reduce the frequency of these murders.
"What we' re missing is Republican willpower to get it done."