The California massacre was carried out on Wednesday by a heavily armed radicalised Muslim husband-and-wife team who, the FBI said, had contacts with terror suspects and who had an armoury of weapons and explosives in their home.

However, despite this and many other indicators of terrorist connections, President Barack Obama said it is still premature to call the killing of 14 people and injury to 21 others at a social services centre in San Bernardino as an act of Islamic terrorism.

Initial findings by investigators said Syed Rizwan Farook, 28, and his wife Tashfeen Malik, 27, were Islamic radicals with a death wish, WND reported. The two originally met online and recently traveled to Saudi Arabia.

The couple opened fire on a holiday party in San Bernardino, firing at least 65 shots when they stormed the conference room at the Inland Regional Center, where 75 to 80 people had gathered for a holiday potluck, the Los Angeles Times reported.

Twelve of the 14 dead and 18 of the 21 injured were county employees, police said.

Hours later, the Muslim couple exchanged gunfire with police on city streets, with bullets ripping into homes and terrifying residents.

In the attack on the party, Farook and Malik used two assault rifles and two semiautomatic handguns, all of which were purchased legally, officials said on Thursday.

When police raided their Redlands home, they saw a virtual armoury of weapons and explosives, including a dozen pipe bombs and thousands of rounds of ammunition, officials said.

The huge cache of weapons and bombs found in the couple's house plus the fact that his getaway vehicle was rented days in advance suggested a level of planning possibly hatched by a terrorist group, investigators said.

"Certainly they were equipped and they could have continued to do another attack. ... We intercepted them," San Bernardino police Chief Jarrod Burguan said at a news conference Thursday.

Moreover, a senior federal government official told the Los Angeles Times that Farook was in contact with suspected extremists and that he communicated with at least one person who is currently being monitored as a potential terror suspect.

Officials said there may be a "deeper terror matrix" behind the San Bernardino shootings.

Yet despite numerous clues that the massacre was carried out by a terrorist group, Obama remained hesitant to call it a terrorist attack.

"At this stage, we do not yet know why this terrible event occurred. We'll get to the bottom of this and be vigilant getting the facts before we issue decisive judgments on how this occurred," Obama said Thursday during an address from the White House.

"It is possible that this was terrorist-related but we don't know," he added, Fox News reported.

"We do know that the two individuals who were killed were equipped with weapons and appeared to have access to additional weaponry at their homes," President Obama said Thursday. "But we don't know why they did it. We don't know at this point the extent of their plans. We do not know their motivations," Obama said.

But the president had more to say on the other possibility that the incident was work-related. "It's also possible that this was workplace related and until the FBI has been able to conduct what is going to be a large number of individuals, until we understand the nature of the workplace relation between the individual and his superiors, because he worked with the organisation where this terrible shooting took place, until all the social media and electronic info has been exploited, we're just not going to be able to answer those questions," he said.

Farook was born in Illinois, and served as an inspector at the Health Department for five years. Malik was born in Pakistan, according to an FBI source.

Farook travelled to Saudi Arabia in 2013 during the Hajj pilgrimage to Mecca and also visited Saudi Arabia in July 2014 for nine days to pick up Malik and bring her to the United States on a K-1 fiance visa, officials said.

Investigators said the couple appeared familiar with weapons and military-style tactics.

Shortly before the shooting, the couple left their young daughter with the child's grandmother in Redlands, saying they had a doctor's appointment, authorities learned.

They headed to the regional centre soon after on a mission to kill as many people as possible.