The suspect behind America’s latest mass shooting has been identified as Ian David Long, who law enforcement officials have accused of killing at least 12 people inside the Borderline Bar & Grill in Thousand Oaks, California.

The gunman, identified by NBC News as a heavily-tattooed, 28-year-old white male, used a .45 calibre handgun and was found dead after storming the bar that’s commonly frequented by country music fans on Wednesday nights. One eye witness said the gunman “was throwing smoke grenades” as he repeatedly fired shots inside the bar, telling reporters, “he just kept firing.”

Details surrounding the suspect began flooding in from neighbours who told ABC News he was a veteran who may have suffered from post-traumatic stress disorder (PTSD). Police said he was an ex-Marine who had multiple run-ins with law enforcement in recent years and likely shot himself Wednesday night in the bar.

The gunman stormed the Borderline Bar & Grill — a venue that has been operating for more than 25 years, according to its website — wearing black while seemingly shooting at random victims. 18-year-old teenagers and above are allowed entry to the venue on Wednesday nights, with those under the legal drinking age receiving black “X” marks on their hands.

An official told the Associated Press the shooter had deployed a smoke device and did not say anything as he opened fire inside of the bar. Hundreds of people immediately fled the scene, with numerous other victims injured while jumping out of windows and ducking under tables, police said Thursday.

Reports arriving out of Thousand Oaks, California painted a picture of the heartbreaking scene. Ron Helus, a local sheriff’s sergeant, died while attempting to rescue victims.

“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 because he went in to save lives, to save other people,” Ventura County Sheriff Geoff Dean told reporters. “I never thought I would see the things around the country that would happen, but I've learned it doesn't matter what community you're in, it doesn't matter how safe your community is, it can happen anywhere.”

Jason Coffmann showed reporters photos of his son, Cody, outside the Thousands Oaks Teen Centre where a gathering point had been established for victims’ families. The father said he had learned his son did not make it out of the bar on Wednesday night.

The Los Angeles Times also reported there were several survivors of the Las Vegas massacre — one of the deadliest mass shootings in American history — inside the bar when the suspect began open firing.

Eye witnesses said the bar was packed on Wednesday night, with more than 100 people enjoying music and drinks — many of them arriving from several large nearby universities.

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Local police reported numerous contacts with the former Marine on Wednesday, including a call that led officers to his home in April of this year. Officials found Mr Long acting "irate and irrationally," however, he was not taken into custody at the time. A mental health crisis team was called to the scene.

The suspect also had prior run-ins with the police, including a traffic incident and another time in which he was the victim of a battery at a bar.

Donald Trump ordered flags at half-staff on Thursday “as a mark of solemn respect for the victims of the terrible act of violence perpetrated in Thousand Oaks, California,” the president said in a statement.

As news broke surrounding the identification of the suspect, a heavy police presence was observed at a home just ten minutes from the Borderline Bar & Grill. It was not immediately clear whether that was the home of Mr Long.