The couple suspected of killing 14 people at a holiday party in California 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 60 miles (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.

The dead and wounded from Wednesday's bloodshed accounted for nearly half of the estimated 75 to 80 people who were in the room where the armed couple opened fire.

San Bernardino Police Chief Jarrod Burguan told a news conference the search of a townhouse leased by the two shooting suspects in the nearby community of Redlands turned up flash drives, computers and cell phones.

Sources in Washington familiar with the investigation said there was no hard evidence of a direct connection between the couple and any militant group abroad, but the electronics would be checked to see if the suspects had been browsing on jihadist websites or social media.

One US government source told Reuters the FBI was examining information indicating that Farook was in contact with individuals who had themselves been under FBI investigation, some from cases already closed. The source also said it was possible that one or more of the Farook contacts under scrutiny were overseas.

But no information has emerged suggesting any ties or contacts between Farook and the Islamic State or other specific militant groups, the source said.

Ideology or workplace anger?

Officials from President Barack Obama to Police Chief 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 (CAIR).

Malik, who had a six-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 travelling on a Pakistani passport.

The couple entered the United States in July 2014 after a trip that included Pakistan, Bowdich said. Farook also visited Saudi Arabia for nine days in the summer of 2014, the kingdom's embassy in Washington said.

The director of the Islamic Center of Riverside, a mosque Farook attended regularly for two years, described him as a devout Muslim who made the pilgrimage to Mecca in Saudi Arabia a few years ago and celebrated his wedding reception at the mosque.

"His degree of faith is very high," the director, Mustafa Kuko, told Reuters. "He was a very quiet person, peaceful, never had an argument with anyone or a dispute."

Kuko said Farook attended morning and evening prayers from 2012 to 2014, when he abruptly stopped coming.

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 that came under attack.

Police cited witness accounts that Farook had been attending the celebration but stormed off in anger, then returned with Malik armed with assault gear and opened fire. Burguan said they sprayed the room with 65 to 70 rounds.

Police officer Mike Madden, one of the first to arrive on the scene, recalled the pandemonium and sheer panic he encountered entering the hall, reeking with the smell of gunpowder and doused in blood and spray from the automatic sprinkler system as fire alarms wailed.

"It was unspeakable, the carnage we were seeing," he recounted at an evening news conference in San Bernardino.

More weapons, more explosives at house

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

At the townhouse, police found another 4,500 rounds, 12 pipe bombs and bomb-making equipment. One bomb was rigged to a remote-control device.

The guns were legally purchased in the United States, said Meredith Davis, a spokeswoman for the Bureau of Alcohol, Tobacco, Firearms and Explosives.

Burguan said Farook bought the two handguns. The rifles were purchased by someone else, who Davis said was not linked to the investigation.

As the FBI-led investigation pressed on Thursday, authorities completed formally notifying the families of the 14 people who died and made their names public.

The victims, all from Southern California, ranged in age from 26 to 60, and most were men, according to the county coroner. All but two of the dead and three of the wounded were county employees.

In addition to sparking further debate on gun control laws, the latest slaying in the United States took place with much of the world on edge following the November 13 attacks in Paris by Islamic State militants that killed 130 people.

Ayloush urged the public not to jump to conclusions about the motives behind the San Bernardino attack. He said he was concerned about a backlash against the Muslim community in view of the rise of Islamic State and some opposition among politicians and the public in the United States over US plans to accept Syrian war refugees.

"We're living in a very difficult time," he told CNN. "There's a lot of Islamophobia out there, a lot of anti-Muslim sentiment, fuelled by pundits here and there trying to blame a whole community for the acts of a few."

About 200 worshipers gathered Thursday night at the Ahmadiyya Muslim Community mosque, San Bernardino's largest, to hold a vigil for victims of the shooting.

Meanwhile, a larger, more diverse crowd estimated by police at about 3,500 packed the San Manuel Stadium, a municipal baseball park downtown, for a candlelight memorial.

"We're here to support our community and be a visual representation of the actual religion of Islam, to show we are caring," said Samar Natour, 16, wearing a pale blue hijab head scarf and holding a sign with a hand-drawn American flag and the message: "We stand with San Bernardino."

Fighting back tears, Anthony Quayle, 33, said he was emotional seeing the community draw together in the face of tragedy.

"I've grown up in this city. I love this city, and I want to be a part of bringing it back together," he said.