SAN BERNARDINO, Calif. (AP) — As many as three gunmen believed to be wearing military-style gear opened fire Wednesday at a Southern California social services center, killing at least 14 people and wounding more than a dozen others, authorities said.

Hours later, police hunting for the attackers riddled a black SUV with gunfire several miles away, and one person lay motionless in the street with a gun nearby, and officers appeared to remove a second person from the vehicle.

It was not immediately unclear if the suspects were the gunmen, though police earlier said the attackers may have fled in a dark SUV.

It was the nation's deadliest mass shooting since the Newton, Connecticut, attack in December 2012 that left 26 children and adults dead.

Police shed no light on a motive for Thursday's massacre, which came just five days after a gunman opened fire at Planned Parenthood in Colorado, killing three.

"They came prepared to do what they did, as if they were on a mission," San Bernardino Police Chief Jarrod Burguan said, noting the attackers carried long guns — which can mean rifles or shotguns.

Witnesses said several people locked themselves in their offices, desperately waiting to be rescued by police, after gunfire erupted at the Inland Regional Center, which serves people with developmental disabilities.

The attack took place in a conference area where the San Bernardino County Department of Public Health was renting space to hold a banquet, said Marybeth Feild, president and CEO of the center. She said the building houses at least 25 employees as well as a library and conference center.

FBI agents and other law enforcement authorities converged on the center and searched room to room for the attacker or attackers, but it was feared that they had escaped.

Ten of the wounded were hospitalized in critical condition, and three were in serious condition, San Bernardino Fire Chief Tom Hannemann said. Police cautioned that the numbers of dead and wounded were early estimates that could change.

No weapons were recovered at the center, though authorities were investigating unidentified items in the building and brought in bomb squads, Burguan said.

San Bernardino police spokesman Sgt. Vicki Cervantes told The Associated Press there were reports from witnesses of one to three gunmen.

As the manhunt went on, stores, office buildings and at least one school were locked down in the city of 214,000 people about 60 miles east of Los Angeles, and roads were blocked off.

Triage units were set up outside the center, 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.

President Barack Obama was briefed on the attack by his homeland security adviser.

He said it was too early to know the shooters' motives, but urged the country to take steps to reduce the frequency of mass shootings. He told CBS that stricter gun laws, including stronger background checks, would make the country safer.

"The one thing we do know is that we have a pattern now of mass shootings in this country that has no parallel anywhere else in the world, and there's some steps we could take, not to eliminate every one of these mass shootings, but to improve the odds that they don't happen as frequently," Obama said.

Terry Petit said his daughter works at the center, where social workers find jobs, housing, transportation and provide programs for people who have disabilities such as autism, cerebral palsy and epilepsy. 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 center. He said she wrote: "People shot. In the office waiting for cops. Pray for us. I am locked in an office."

Sherry Esquerra was searching for her daughter and son-in-law, both of whom work at the center. She said her daughter helps "very disabled" children and "gets all the services she possibly could for these kids."

"I just don't understand why somebody would come in and start shooting," Esquerra said. She last saw her daughter at Thanksgiving and planned to see her Friday. When she calls her phone now, "Nothing. I just get her message. Straight to voicemail."

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 social services center has two large buildings that require a badge to get in, said Sheela Stark, an Inland Regional Center board member. However, the conference room where many public events take place — including the banquet on Wednesday — is usually left open when visitors are expected.

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Contributing to this report were Brian Melley, John Antczak, Christopher Weber, John Rogers, Christine Armario, Gillian Flaccus and Sue Manning in Los Angeles; Amy Taxin in San Bernardino; Alina Hartounian in Phoenix; Michael Sisak in Philadelphia; and Hannah Cushman in Chicago.