LOS ANGELES (Reuters) - San Bernardino shooter Syed Rizwan Farook, 28, had contact with people from at least two militant organisations overseas, including the Al-Qaeda-affiliated Nusra Front in Syria, The Los Angeles Times reported on Friday (Dec 4), citing a federal law enforcement official.

The official described "some kind" of contact between Farook and people from the Nusra Front and the radical al Shabaab group in Somalia, the Times reported. It is unclear what type of contact or with whom, the newspaper said.

Officials noted on Friday that his wife was believed to have pledged allegiance to a leader of the militant group Islamic State in Iraq and Syria (ISIS).

Tashfeen Malik, 29, a native of Pakistan who lived in Saudi Arabia for more than 20 years, and her husband were killed 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 100km 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.

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

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

Mr Bowdich said the FBI hoped examination of data retrieved from the 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 Mr Bowdich said they may have been planning an additional attack.

One startling disclosure came from social media network Facebook, which confirmed that comments praising ISIS 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 ISIS leader Abu Bakr al-Baghdadi.

Asked about a reported Facebook post by Malik on the day of the attack pledging loyalty to Islamic State, Mr 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 ISIS, 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.

ISIS, which has seized large swathes 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 killers were "part of an organised larger group, or form part of a cell", Mr Comey said. "There is no indication that they are part of a network."

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

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

Farook family attorneys, holding a news conference in Los Angeles, denied there was any evidence that either the husband or wife harbored 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 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 six-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.

Attorney Mohammad Abuershaid called the media's intrusion an"invasion of privacy". saying news teams were "taking unacceptable pictures of private things in the house, such as family photos and clothing".

Child welfare authorities have taken custody of the baby, and Farook's relatives were seeking return of the infant, hoping to place her with Farook's older sister, Abuershaid said.

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 the 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.

Mr 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," he 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.

The San Bernardino attacks have raised concerns among Muslim-Americans of an anti-Islamic backlash.