San Bernardino - One or more gunmen believed to be wearing military-style gear opened fire Wednesday at a California social services centre, killing an undisclosed number of people and wounding several others, authorities said.
Federal and local law enforcement authorities converged on the centre and searched for the shooter or shooters.
"There are multiple casualties, and there are some confirmed fatalities. I don't have numbers at this point," San Bernardino police spokesperson Sergeant Vicki Cervantes told KCBS-TV.
She said that "it appears there is more than one shooter", and that they were believed be wearing military-style gear.
Witnesses said several people locked themselves in their offices, desperately waiting to be rescued by police, after gunfire erupted at the Inland Regional Centre, which serves people with developmental disabilities.
The shooting - less than three weeks after the Paris attacks that left 130 people dead - immediately raised the spectre of terrorism, especially after the San Bernardino police said there may have been more than one gunman, and President Barack Obama was briefed on the situation by his homeland security adviser.
But police shed no light on a motive.
As the manhunt went on, stores, office buildings and at least one school were locked down in the city of 214 000 people about 100km east of Los Angeles.
Triage units were set up outside, and people were seen being wheeled away on stretchers. Others walked quickly from a building with their hands up. They were searched by police before being reunited with loved ones.
Terry Petit said his daughter works at the centre, and he got a text from her saying she was hiding in the building after hearing gunshots. Petit choked back tears as he read the texts for reporters outside the centre. He said she wrote: "People shot. In the office waiting for cops. Pray for us. I am locked in an office."
Marcos Aguilera's wife was in the building when the gunfire erupted. He said a shooter entered the building next to his wife's office and opened fire.
"They locked themselves in her office. They seen bodies on the floor," Aguilera told KABC-TV, adding that his wife was able to get out of the building unharmed.
The president and CEO of the social services centre, Maybeth Feild, said the shooting took place in a conference area that an outside group was renting on Wednesday. She said she died not know the identity of the group.
She said that the building houses at least 25 employees as well as a library and conference centre.
Loma Linda University Medical Centre received four patients in its trauma center and expected three more, spokesperson Briana Pastorino said. She did not know their conditions.