Police in San Bernadino, California are searching for up to three gunmen who killed at least 14 people at a social services centre.
Officers said there were also 14 wounded in the attack at the Inland Regional Center, which helps adults with disabilities. The gunmen, in military-style clothing, entered the building and opened fire before fleeing in a black SUV.
Witnesses said people locked themselves in offices as the gunfire rang out.
According to the the Centre's president, Maybeth Field, said the attack took place in a conference area where the San Bernardino County Department of Public Health was holding a social event. FBI officials said they do not yet know if it was a terrorist attack but local police said it might have been domestic terrorism.
The shooting happened less than a week after a shooting at a Planned Parenthood clinic in Colorado left three people dead and nine people wounded.