NEW YORK - US authorities are investigating the motive behind a mass shooting that left 14 people dead and another 17 wounded at a social services centre hosting a Christmas party in Southern California.

Islamic organisations across the United States expressed horror at the attack after a local Muslim couple was identified as suspect in Wednesday’s shooting.

Syed Rizwan Farook, 28, and Tashfeen Malik, 27, who had a 6-month-old daughter together, were killed in a shootout with police after the bloodshed at the Inland Regional Center in the city of San Bernardino.

Before the attack, US-born Farook, whose parents are of Pakistani origin, joined colleagues at the annual holiday party for the San Bernardino County Public Health Department, where he has worked for five years as an environmental inspector, officials were quoted as saying. He had attended the same party the year before, and he did not appear out of place.

Soon, however, he stormed out in anger, according to accounts carried by news media. The nature of the dispute was not clear, but when he returned with his wife, whose is also of Pakistani origin, both of them dressed in tactical gear and carrying assault rifles, officials said. That level of preparation is among the factors investigators are weighing as they examine a motive for the attack, the report said.

The attackers were dressed in military-style gear and carried assault weapons as they burst into the auditorium where the shooting took place, at the campus of the agency, according to media reports.

The couple also placed several bombs at various locations, which police detonated. In addition to the 14 dead, the shooting left at least 17 people wounded, including 10 in critical condition.

Meredith Davis, a spokeswoman for the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), said the two rifles and two handguns recovered at the scene of the shootout were bought legally in the United States.

Two of the guns were purchased by someone “associated with this investigation,” while the buyer of the other two was not linked to the investigation, she said. The long guns were 223-caliber and their ammunition can go through protective vests and walls, Davis said.

San Bernardino Police Chief Jarrod Burguan said the motive remained unclear in the most recent US mass shooting . He and David Bowdich, an assistant regional FBI director, both said it had not been determined if terrorism was a motive.

Police were conducting a house-to-house search in the area where the third suspect was apprehended and completed the search early in the evening, when the “shelter-in-place” warning to residents was lifted, according to police.

Citing law enforcement officials, CNN reported that Farook was apparently radicalised, though other factors may have played a role in his motive.

CNN said Farook had been in touch over the telephone and via social media with more than one international terrorism subject who was being investigated by the FBI.

Hussam Ayloush, executive director of the Los Angeles chapter of Council on American-Islamic Relations (CAIR), said the couple left their baby with family Wednesday morning and never returned.

“We condemn this horrific and revolting attack and offer our heartfelt condolences to the families and loved ones of all those killed or injured,” said Ayloush. “The Muslim community stands shoulder to shoulder with our fellow Americans in repudiating any twisted mindset that would claim to justify such sickening acts of violence.”

Farhan Khan, who is married to Farook’s sister, spoke to reporters at the Anaheim CAIR office.

Khan said he last spoke to Farook about a week ago, adding he had “absolutely no idea why he would do this. I am shocked myself.” Khan says other family members asked him to speak at the news conference, and to express their sadness over the shootings.

The Islamic Circle of North America (ICNA), a New York-based Muslim body, said it was “appalled” at Wednesday’s mass shooting . “We support President Obama’s call for ‘common sense gun safety laws’ to end these senseless shootings plaguing our nation.”

Meanwhile, Farook’s father, quoted by the New York Daily News tabloid, described his son as a devout Muslim.

“He was very religious. He would go to work, come back, go to pray, come back. He’s Muslim.”

According to press reports, Farook drew an annual salary of $70,000.

Muzammil Siddiqi, the religious director of the Islamic Society of Orange County, also condemned the killings saying they were contrary to the teachings of Islam.”We have condemned all violence everywhere because human life is precious,” he said. “And we respect and honour human life.”

Meanwhile, Bowdich, assistant director of the FBI’s field office in Los Angeles, says one possibility is workplace violence and another is “terrorism.”

The mass shooting is the deadliest US gun violence since the massacre at Sandy Hook Elementary School in Newtown, Connecticut, in December 2012, in which 27 people, including the gunman, were killed.

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 mass shootings, including stricter gun laws and stronger background checks.

Burguan said all that was known about the shooters is that “these people came prepared.”

Reuters adds: The couple left the baby with Farook’s mother in the nearby city of Redlands early Wednesday. They told her they were going to attend a doctor’s appointment for the wife.

Officials said police investigating the shooting went to a house in Redlands on Wednesday afternoon, and saw the couple take off in a black SUV. Police pursued the car to San Bernardino, where the gunbattle ensued that left Farook and Malik dead.