The couple suspected of killing 14 people at a party in California had more than 6,000 rounds of ammunition and a dozen pipe bombs on them and elsewhere, authorities said as they sought to determine if the pair had links to Islamic militant groups.
Syed Rizwan Farook, 28, and Tashfeen Malik, 27, his wife and mother of his six-month-old daughter, were killed in a shootout with police five hours after yesterday's massacre at the Inland Regional Centre social services agency in the city of San Bernardino.
21 people were also wounded in the shooting.
San Bernardino Police Chief Jarrod Burguan said at a news conference that a search of a townhouse in nearby Redlands believed used by Farook and Malik yielded flash drives, computers and mobile phones.
Officials in Washington familiar with the investigation said so far there was no hard evidence of a direct connection between the shooters and any militant group abroad but that the electronics would be checked to see if the couple had been browsing on jihadist websites or social media.
CNN, citing law enforcement sources, said Farook had been "radicalised" and had been in touch through telephone and social media with more than one international terrorism suspect who was being investigated by the FBI.
San Bernardino Police Chief Jarrod Burguan speaks about dead chief suspect in mass shooting https://t.co/BId3NOIg6L
— RTÉ News (@rtenews) December 3, 2015
Farook, a US citizen born in Illinois, was the son of Pakistani immigrants.
State Department spokesman Mark Toner said Malik had been granted a 'fiancee visa' to come to the US with Farook and that all applicants for the visa are fully screened.
Witnesses said Farook had been at the work holiday party, before leaving - possibly after a dispute - and returning with Malik and opening fire.
Mr Burguan said they sprayed the room with 65 to 70 rounds.
"It is possible that this was terrorist-related. But we don't know," US President Barack Obama said.
"It is also possible that this was workplace-related," he added.
He said the FBI was taking over the investigation.
David Bowdich, the FBI assistant director in Los Angeles, said the couple had entered the US together in July 2014 after a trip that included Pakistan and perhaps other countries.
Farook did not have a criminal record, Mr Burguan said.
Mr Burguan said the couple had two assault-style rifles, two handguns and 1,600 rounds of ammunition in their vehicle when they were killed.
At the townhouse, police found another 4,500 rounds, 12 pipe bombs and bomb-making equipment.
The guns were legally purchased in the US, said Meredith Davis, a spokeswoman for the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF).
In addition to sparking further debate on gun control laws, the latest mass slaying in the US came with much of the world on edge following the 13 November attacks in Paris by so-called Islamic State militants that killed 130 people.
"We're going to have to, I think, search ourselves as a society to make sure that we can take the basic steps that would make it harder - not impossible but harder - for individuals to get access to weapons," Mr Obama said at the White House.
He ordered flags to be flown at half-mast in memory of the shooting victims.
Ten people remain at two hospitals - two in critical but stable condition, three in fair condition and five in stable condition, the hospitals said.
The San Bernardino rampage was the deadliest US shooting incident since the December 2012 massacre at Sandy Hook Elementary School in Newtown, Connecticut, in which 27 people, including the gunman, were killed.
There have been more than 350 mass shootings in the US this year.
Brother in law of Syed Rizwan Farook, the main suspect in the San Berardino shooting, speaks to media https://t.co/PUA15IjFSc
— RTÉ News (@rtenews) December 3, 2015
Farook's brother-in-law, Farhan Kahn, came forward at a press conference to voice his shock at the notion his brother could have committed mass murder.
"I have no idea why he would do that," a visibly shaken Mr Khan told reporters, adding that he had last spoken to Farook about a week ago.
"I am in shock that something like this could happen," said Mr Khan, who is married to Farook's sister. "I am very sad that people lost their lives."
