On Tuesday morning California time, adults, infants and senior citizens gathered at the Inland Regional Centre to take part in the annual Christmas party.

There was singing and dancing at the San Bernardino disability facility, as the group marked the start of the holiday season – a time spent with family and friends.

In a video (below) posted by the centre on its Twitter page, people could be seen in wheelchairs swaying to Kool and the Gang’s hit-song Celebration.

But, by the next morning, while a group of government workers were also reportedly attending end of year celebrations, three suspected gunmen stormed the facility in a brutal shootout – killing 14 people and injuring 14 others.

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The Inland Regional Centre is a multi-service state-run agency with almost 700 employees that connect clients with services, to make them more included in the community and live more independently.

The centre opened in 1971 and is one of 21 run by not-for-profit groups across California.

The California Department of Developmental Services told the Washington Post that the Inland Regional Centre provided services to people with disability, like like autism and cerebral palsy, such as counselling, assessment and diagnoses, early intervention for infants, education, training, legal assistance, and connecting people to other community services.

Workers from the centre huddle together after they were evacuated. Photo: Getty

Now, the facility sits bloodied and abandoned as police investigate the attack.

A mother from the centre, Kristine Metcalf, told the Washington Post that her 11-year-old daughter would never have learnt to walk and talk without the help of its staff.

Her daughter, Kimie, was born premature and with cerebral palsy, is blind in her right eye and has been diagnosed with a rare form of blood cancer.

“She is the product of Inland Regional Center, she would not be where she is today without it,” Ms Metcalf said.

After hearing about the mass shooting at the facility, Ms Metcalf told the newspaper she had been franticly trying to contact some of the employees, but could not reach them.

She was at a loss with why people would want to harm these good workers who had done so much for her family and thousand of others.

“I can’t tell people enough how their services have made a life-changing difference,” she said.

Olivia Navarro waits to see her daughter after a mass shooting. Photo: Getty

“The people who work there are amazing. it was really amazing to see and meet everybody.”

On Thursday morning (AEDT), the centre’s website was down ‘for security measures’ but its Facebook page said its mission was to “help provide each individual with a service system that helps identify and eliminate barriers for individuals with developmental disabilities and their families so they can closely live a typical lifestyle.”

Sergeant Vicki Cervantes told reporters that the centre hosted “some kind of function today, an event, and during that event is when multiple shooters came in and just started shooting”.

She said “multiple shooters” targeted a function in the conference centre at the site.

By about 12.30pm (AEDT) on Thursday, police confirmed two suspects, a male and a female, were dead. They said a third suspect has been detained.

Watch the video of the Inland Regional Center’s Christmas party, the day before the shooting: