SAN BERNARDINO: The FBI is investigating the massacre of 14 people in California by a married couple armed with assault rifles as an "act of terrorism," officials said on Friday, noting the wife was believed to have pledged allegiance to a leader of the militant group Islamic State.

Tashfeen Malik, 27, a native of Pakistan who lived in Saudi Arabia for more than 20 years, and her US-born husband, Syed Rizwan Farook, 28, were killed in a shootout with police hours after the Wednesday attack during 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 Sept. 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.

He pointed, in particular, to investigators' discovery of two crushed cell phones left by the couple in a trash can near their rented townhouse.

Bowdich said the FBI hoped examination of data retrieved from the cell phones 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 Inc (FB.O) 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, U.S. government sources said. Islamic State, which has seized large swaths of Syria and Iraq, claimed responsibility for the Nov. 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 accused shooters 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."

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, worked 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. The landlord later asked media to leave.

PROBE EXTENDS TO PAKISTAN

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

"I only found out about this tragedy today when some intelligence officials contacted me to ask me about my links with Tashfeen," Malik's uncle, Javed Rabbani, said in an interview. "I had heard in the news that this tragedy had taken place but I could never even imagine that it would be someone from my family. Of course, we are in shock."

He 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.

Christian Nwadike, who worked with Farook for five years, told CBS that his co-worker had been different since he returned from Saudi Arabia.

"I think he married a terrorist," Nwadike said.

Twenty-one people were wounded in the attack, the worst gun violence in the nation since the December 2012 shooting at Sandy Hook Elementary School in Newtown, Connecticut.

Farook had not been under surveillance by the FBI or any other law enforcement agency, Bowdich said, adding that there was no evidence that additional threats lingered following the shooting.