A young church worker who 'loved serving people' has been confirmed as one of the victims of Wednesday's mass shooting at a bar in Thousand Oaks, California.

Noel Sparks, 21, was a member of staff at Calvary Community Church in Westlake Village.

Church pastor Shawn Thornton said: 'She's now stepped into the presence of the Lord and it's been heavy for us. There have been some tears shed, we've had some tough conversations.'

The church has held a prayer vigil following the shooting and is also hosting support meetings for those struggling to come to terms with what has happened.

Friend Sarah Penrose, who attended the church's Bible study together with Sparks, said she loved serving others.

'As we say at Calvary often, she lived and loved like Jesus,' Penrose said.

'She was genuinely caring, and loved serving people. She definitely left a legacy of selfless servitude.'

Authorities have identified the gunman as 28-year-old Ian Long, a former Marine.  Twelve people were killed when he stormed the Borderline Bar during a student night on Wednesday before turning the gun on himself. 

Police had been called to Long's home in April over a disturbance where officers described him as being 'irate' but decided not to detain him. 

The authorities have not yet established a motive for Wednesday's shooting, although CNN reports that the authorities have found a social media post believed to have been made by the gunman around the time of the attack saying: 'I hope people call me insane.  wouldn't that just be a big ball of irony? Yeah.. I'm insane, but the only thing you people do after these shootings is 'hopes and prayers'.. or 'keep you in my thoughts'... every time... and wonder why these keep happening...'