Witnesses say the gunman was casually sitting down when he opened fire with a beer in his hand. Picture: AP

It comes as tributes flow in for Monique Clark, a 35-year-old mother-of-three, who was killed in the random attack at an apartment complex in San Diego.

Nick Cannon, ex-husband of singer Mariah Carey, was a childhood friend.

He posted an old picture on Instagram, commenting that “tears can’t express the pain and shock” of her death.

Monique Clark was killed in San Diego, leaving behind three daughters aged 2, 11 and 13.Source:Facebook

“My heart hurts with great sadness tonight ... such a beautiful spirit with an infectious smile. I have nothing but wonderful memories of this Angel,” he wrote.

“She was always smiling. She was always with her kids and everyone else’s kids. She was like a mother to them all,” said Vincent Howard, who knew Ms Clark since school.

He said she also helped homeless people, and volunteered at food drives.

“The grieving process is fresh, raw, and real,” Ms Clark’s mother, Michelle Fuget, wrote on a GoFundMe page to raise money for her daughters, aged two, 11 and 13.

“My granddaughters lost their mother, I lost my daughter, her siblings lost their sister, and countless other family members and close friends have lost someone who was taken from us too soon due to a senseless cowardly act of violence.”

Demetrius Griffin said the gunman casually sat by the pool while he opened fire on guests. "He didn't stand up ... Even when he reloaded his magazine, he sat in his chair.” Picture: Julie WatsonSource:AP

Police have suggested the gunman, 49-year-old Peter Selis, was devastated after a recent break up when he opened fire on guests.

They say he called his ex-girlfriend during the attack so she could hear the screams as shot Ms Clark and six other people.

Survivors, on the other hand, believe racism could have been a factor as most guests attending the party were African-American.

Navy Lieutenant Lauren Champan, one of the survivors, said the gunman deliberately avoided shooting at least one white guest.

“I’m a realistic person and we need to acknowledge race was a factor,” she said.

“Maybe it wasn’t everything, but it was definitely something. We need to get that dialogue open. Yes, he was bankrupt. Yes, he had relationship issues, but I think we need to acknowledge the possibility of bias.”

She later spoke to the woman who escaped, who said the gunman told her “you can either leave or you can stay here and die”.

The 49-year-old gunman was killed after turning his weapon on police officers. Picture: Gregory BullSource:AP

A white woman was allowed to escape the pool area when the gunman opened fire.Source:AP

However, authorities say Mr Selis made no statements and showed no behaviour to suggest his actions were racially-motivated.

“I’m not suggesting that maybe that didn’t happen but we have zero information suggesting that was the case,” San Diego Assistant Chief Brian Ahearn told AP.

Mr Selis was killed in a shootout with police, after turning his handgun on a sergeant and two other officers.

The other victims are all expected to survive.

The Associated Press contributed to this report.