Suspects had acquired thousands of rounds of ammunition, plus rifles, 12 pipe bombs

The couple suspected of killing 14 people at a holiday party in California had amassed thousands of rounds of ammunition and a dozen pipe bombs, authorities said on Thursday as they sought clues to the pair's motives and whether they had links to Islamist militants.

Syed Rizwan Farook, 28, and his wife, Tashfeen Malik, 27, were killed in a shootout with police five hours after Wednesday's massacre at the Inland Regional Center social services agency in the city of San Bernardino, about 100 km east of Los Angeles.

Twenty-one people were wounded in the attack, which ranks as the deadliest instance of US gun violence since the December 2012 shooting at Sandy Hook Elementary School in Newtown, Connecticut, in which 27 people were killed.

A US intelligence official said on Thursday that Farook had been in contact with known Islamic extremists on social media, and police said he and his wife had enough bullets and bombs to slaughter hundreds when they launched their deadly attack.

The details emerged as investigators tried to determine whether the rampage was terrorism, a workplace grudge or some combination.

Officials from US President Barack Obama to San Bernardino Police Chief Jarrod Burguan said the attack may have been motivated by extremist ideology, but that questions of motive remained unanswered.

"It is possible that this was terrorist-related. But we don't know," Obama told reporters. "It is also possible that this was workplace-related."

Farook, a US citizen born in Illinois, was the son of Pakistani immigrants, according to Hussam Ayloush, who heads the Los Angeles area chapter of the Muslim advocacy group Council on American-Islamic Relations.

Malik, who had a 6-month-old daughter with Farook, was a Pakistani native living in Saudi Arabia when they married, Ayloush said.

David Bowdich, FBI assistant director in Los Angeles, said Malik was admitted to the United State on a K-1 "fiancee visa" and was traveling on a Pakistani passport.

Farook, who according to Burguan had no criminal record, worked as an inspector for San Bernardino County Department of Environmental Health, the agency throwing the holiday party.

Wearing black tactical gear and wielding assault rifles, Farook and Malik sprayed as many as 75 rounds into a room at a social service center for the disabled, where about 75 of Farook's co-workers had gathered. Farook had attended the event but slipped out and returned in battle dress.

The couple died in a furious gunbattle in which they fired 76 rounds, while 23 law officers unleashed about 380.

Burguan said the couple had two assault-style rifles, two semi-automatic handguns and 1,600 rounds of ammunition in their rented SUV when they were killed.

At the town house, police found another 4,500 rounds, 12 pipe bombs and bomb-making equipment.

Reuters - AP