The FBI is investigating the fatal shooting of 14 people in California by a married couple as an "act of terrorism," officials said on Friday.

While the FBI said it lacked evidence the couple belonged to a larger organisation of extremists, the Los Angeles Times cited a federal law enforcement source in reporting that the husband, Syed Rizwan Farook, 28, had contact with at least two militant groups overseas, including the al Qaeda-affiliated Nusra Front in Syria.

Both the US-born husband and his spouse, Tashfeen Malik, 29, a native of Pakistan who lived in Saudi Arabia for more than 20 years, died in a shootout with police hours after Wednesday's attack on a holiday party at the Inland Regional Center social services agency in San Bernardino, about 60 miles (100 km) east of Los Angeles.

If the mass shooting proves to have been the work of people inspired by Islamist militants, as investigators now suspect, it would mark the deadliest such attack in the United States since September 11, 2001.

Federal Bureau of Investigation officials said mounting signs of advanced preparations, the large cache of armaments amassed by the couple and evidence that they "attempted to destroy their digital fingerprints" helped tip the balance of the investigation.

"Based on the information and the facts as we know them, we are now investigating these horrific acts as an act of terrorism," David Bowdich, assistant director of the Federal Bureau of Investigation's Los Angeles office, said at a news conference.

Bowdich said the FBI hoped examination of data retrieved from two smashed cellphones and other electronic devices seized in the investigation would lead to a motive for the attack.

The couple had two assault-style rifles, two semi-automatic handguns, 6,100 rounds of ammunition and 12 pipe bombs in their home or with them when they were killed, officials said. And Bowdich said they may have been planning an additional attack.

Pledge of allegiance

One startling disclosure came from social media network Facebook, which confirmed that comments praising Islamic State were posted around the time of the mass shooting to a Facebook account established under an alias by Malik. However, it was uncertain whether the comments were posted by Malik herself or someone with access to her page.

A Facebook spokesman said the profile in question was removed by the company on Thursday for violating its community standards barring promotion or praise for "acts of terror". He declined to elaborate on the material.

But CNN and other news media outlets reported that Malik's Facebook posts included a pledge of allegiance to Islamic State leader Abu Bakr al-Baghdadi.

Asked about a reported Facebook post by Malik on the day of the attack pledging loyalty to Islamic State, Bowdich said, "I know it was in a general timeline where that post was made, and yes, there was a pledge of allegiance."

While Malik and her husband may have been inspired by Islamic State, there was no evidence the attack was directed by the militant group, or that the organisation even knew who they were, US government sources said. Islamic State, which has seized large swaths of Syria and Iraq, claimed responsibility for the November 13 attacks in Paris in which gunmen and suicide bombers killed 130 people.

Speaking to reporters separately in Washington, FBI Director James Comey said the investigation pointed to "radicalisation of the killers and of potential inspiration by foreign terrorist organizations."

But no evidence has been uncovered yet suggesting the killers were "part of an organised larger group, or form part of a cell," Comey said. "There is no indication that they are part of a network."

Bowdich said neither Farook nor Malik had been under investigation by the FBI or other law enforcement agency prior to Wednesday.

And none of the contacts federal agents have since discovered between the couple and the subjects of other FBI inquiries "were of such a significance that it raised these killers up onto our radar screen," Comey said.

Citing an unnamed federal law enforcement official, the Los Angeles Times reported late on Friday that Farook had "some kind" of contact with people from the Nusra Front and the radical Shabab group in Somalia. But the nature of that contact and with whom was unclear, the Times said.

The official also was quoted by the Times as saying a key question in the investigation "is if they had any weapons or terror training in Pakistan."

Family unaware of extremist views

Farook family attorneys, holding a news conference in Los Angeles, denied there was any evidence that either the husband or wife harboured extremist views.

"She was like a typical housewife," lawyer David Chesley said, describing Malik as "caring, soft-spoken" and a devout Muslim who prayed five times a day, chose not to drive and "kept pretty well isolated".

She spoke broken English and her primary language was Urdu, he said, adding, "She was very conservative."

They said Farook, too, largely kept to himself, had few friends and said co-workers sometimes made fun of his beard.

Farook, born in Illinois to Pakistani immigrant parents, earned over $50,000 a year as an inspector for the San Bernardino County Department of Environment Health, the agency whose holiday party he and Malik are accused of attacking on Wednesday.

Investigators are looking into a report that Farook had an argument with a co-worker who denounced the "inherent dangers of Islam" prior to the shooting, a US government source said.

The couple's landlord in the town of Redlands opened their townhouse to media on Friday, leading to a flurry of reporters and camera crews surveying the scene after the FBI had finished conducting its 24-hour-long search of the premises.

The landlord later asked media to leave the home.

The couple and their 6-month-old daughter shared the home with Farook's mother, in whose care they left the child on Wednesday morning, saying they had a doctor's appointment, according to family representatives.

Child welfare authorities have taken custody of the baby, and Farook's relatives were seeking return of the infant, Abuershaid said.

Pakistani intelligence officials have contacted Malik's family in her homeland as part of the investigation, a family member said.

Malik's uncle, Javed Rabbani, said in an interview with Reuters that the family was "in shock". He also said his brother, Malik's father, had become considerably more conservative since moving with his family to Saudi Arabia a quarter century ago.

Tashfeen Malik had not come to the attention of authorities while living in Saudi Arabia, according to a source close to the Saudi government. She had moved back to Pakistan five or six years ago to study pharmacy, Pakistani officials said.

The San Bernardino attacks have raised concerns among Muslim-Americans of an anti-Islamic backlash. Two days after the San Bernardino attacks, a Reuters/Ipsos poll showed 51 per cent of Americans view Muslims living in the United States the same as any other community, while 14.6 per cent were generally fearful of Muslims.