SAN BERNARDINO, Calif. — Authorities on Thursday were trying to learn why a couple left behind their infant daughter and carried out a 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 gunman’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 attacker 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.

Police and federal agents for a second day searched a home in neighboring Redlands, about 7 miles from the massacre at Inland Regional Center. A black sedan parked outside was also searched early Thursday. The home is where officers initially saw a vehicle matching the description of the attackers’ SUV in the hours before the final gunbattle that killed them. A bomb squad on Wednesday swept the home with robots.

Police didn’t immediately say if the couple lived at the home. Public records show it is a possible residence of a family member of Farook.

Residents told KABC-TV Redlands is a sleepy little town and expressed shock that the killers might be their neighbors.

The attackers invaded the 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.