SAN BERNARDINO, Calif. – Authorities on Thursday were trying to learn why a couple left behind their infant daughter and carried out a shooting rampage that left 14 people dead and seriously wounded more than a dozen others in one of the nation’s worst mass shootings.

The pair were killed in a shootout with police hours after they carried out their precision assault. The possible motive for the attack included workplace violence or terrorism.

Wednesday’s shooting happened at a social services center for the disabled where the suspect’s colleagues with the San Bernardino County Department of Public Health were renting space for a celebration. It was the nation’s deadliest mass shooting since the attack at a school in Newtown, Connecticut, three years ago that left 26 children and adults dead.

San Bernardino Police Chief Jarrod Burguan identified one dead suspect as Syed Rizwan Farook, 28, the other as Tashfeen Malik, 27, his wife or fiancee. Burguan said Farook was born in the United States; the chief said he did not know Malik’s background.

The attackers invaded the Inland Regional Center about 60 miles east of Los Angeles around 11 a.m., opening fire in a conference area where county health officials were having an employee banquet, said Marybeth Feild, president and CEO of the nonprofit center.

“They came prepared to do what they did, as if they were on a mission,” Burguan said.

Farook attended the event before leaving – and returning to kill.

Co-worker Patrick Baccari said he was sitting at the same table as Farook, who suddenly disappeared, leaving his coat on his chair. Baccari said when the shooting started, he sought refuge in a bathroom and suffered minor wounds from shrapnel slicing through the wall.

Baccari described Farook as reserved and said he showed no signs of unusual behavior. Earlier this year he traveled to Saudi Arabia and returned with a wife, later growing a beard, Baccari said.

The FBI is investigating several possible motives, including workplace violence and terrorism, according to David Bowdich, assistant director of the bureau’s Los Angeles office. He did not elaborate.

Farook was a restaurant inspector for the health department, according to public records. Police chief Burguan said he had been a county employee for five years.

The couple dropped off their 6-month-old daughter with relatives Wednesday morning, saying they had a doctor’s appointment, Hussam Ayloush, executive director of the Council on American-Islamic Relations, said after talking with family. Farhan Khan, who is married to Farook’s sister, told reporters he last spoke to his brother-in-law about a week ago. He said he was in shock, condemned the violence, and had “absolutely no idea why he would do this.”

About four hours after the morning carnage, police hunting for the killers riddled a black SUV with gunfire in a shootout 2 miles from the social services center in this Southern California city of 214,000 people. Farook and Malik were found with assault rifles and semi-automatic handguns, and were wearing “assault-style clothing” with ammunition attached, authorities said.

Television news helicopters captured the aftermath, as officers slowly approached the mangled SUV.

In the morning, as the day’s first bursts of gunfire echoed through the large three-building complex, some people locked themselves in offices, desperately waiting for police and texting or making hushed phone calls to loved ones.

“People shot. In the office waiting for cops. Pray for us. I am locked in an office,” Terry Petit’s daughter, who works at the center, texted him.

Petit choked back tears as he read his daughter’s words for reporters outside the center, where social workers find jobs, housing and transportation and provide other services to people with disabilities such as autism, cerebral palsy and epilepsy.

Olivia Navarro said her daughter, Jamile Navarro, a case manager at the social service center, called her and whispered that she was hiding in a locked room.

“I said, ‘All right, I’ll be there, turn off the lights, don’t make a sound,'” Navarro said. “And that was it.”

Her daughter survived.

That the violence happened at a place dedicated to helping people with developmental disabilities – even if they were not targeted – made it even harder for some to comprehend.

“These are all disabled kids, very disabled,” said Sherry Esquerra, who was searching for her daughter and son-in-law, both of whom work at the center. “She gets all the services she possibly could for these kids. So I just don’t understand why somebody would come in and start shooting.”

According to its web page, the center has a client base of more than 30,000 people and their families. It is a privately run nonprofit, the largest of its kind in California with about 670 employees.

FBI agents and other law enforcement authorities converged on the center and searched room to room for the attackers. Triage units were set up outside, and people were wheeled away on stretchers.

Seventeen people were wounded, according to authorities. Ten were hospitalized in critical condition, and three were in serious condition, Fire Chief Tom Hannemann said.