The mass shooting in California is being investigated as "an act of terrorism," the FBI said, amid reports the female assailant had pledged allegiance to the Islamic State group on Facebook.

The developments in the probe come two days after 14 people were killed and 21 others wounded at a year-end office party in San Bernardino -- the deadliest mass shooting in the United States since the Newtown school massacre in 2012.

US-born Syed Farook, 28, and his 27-year-old Pakistani wife Tashfeen Malik were killed in a firefight with police hours after the attack, leaving investigators to comb through their belongings to try to determine a motive.

"We have uncovered evidence that has led us to learn of extensive planning," David Bowdich, the assistant FBI director in charge of the Los Angeles office, told reporters.

"There's a number of pieces of evidence that has essentially pushed us off the cliff to say we are now investigating this as an act of terrorism."

He added that investigators were examining a Facebook posting in which Malik is believed to have pledged allegiance to IS leader Abu Bakr al-Baghdadi, made around the time of Wednesday's attack.

"I'm aware of the Facebook posting," Bowdich said. "And yes, there was a pledge of allegiance."

This image obtained from the San Bernardino County Sheriff, shows weapons carried by the suspects in the December 2, 2015, mass shooting during a holiday party at the Inland Regional Center in San Bernardino, California. Photo: AFP

FBI director James Comey said the probe so far had shown that the couple were radicalized and inspired by "foreign terrorist organizations," but were not part of a larger group or cell.

Bowdich said investigators were scrutinizing evidence seized at the couple's home and elsewhere to determine what prompted the carnage.

"They attempted to destroy their digital fingerprints and we found cell phones that were actually crushed in a nearby trash can," Bowdich said.

A pro-IS news agency, Aamaq, on Friday said the mass shooting was perpetrated by sympathizers of the radical group, which has urged followers in the United States and elsewhere to carry out lone wolf attacks.

'Mind-boggling'

Relatives of Farook and Malik were at a loss to explain how the couple, who had an infant girl and seemed to be living a normal life, could have committed mass murder.

"I can never imagine my brother or my sister-in-law doing something like this. Especially because they were happily married, they had a beautiful six-month-old daughter," Farook's sister Saira Khan told CBS News.

"It's just mind-boggling why they would do something like this."

The family's attorneys said while the couple were devout Muslims, there was no hint they had became radicalized.

"There is no sign that the alleged shooters belonged to a larger organization or a terrorist cell," David Chesley, one of two attorneys representing the family, told a news conference.

Tashfeen Malik and her husband Syed Farook. Photo: AP

Attorney Mohammad Abuershaid said few people came in contact with Tashfeen, who wore the full-face veil and was very soft-spoken.

"The women (in the family) communicated with her. Farook didn't want anyone else to talk to her," he said.

He added that when Farook's family members came to visit the women sat in one room and the men in another.

Abuershaid said Farook had met Malik in 2013 through an online marriage site and then had travelled to Saudi Arabia, where she lived, to meet her family. He said Farook traveled again to Saudi Arabia last year where the couple married.

One of Farook's colleagues said he was convinced Malik had radicalized her husband.

Abuershaid and Chesley said Farook had apparently been teased by colleagues at the county health department, where he had worked as an inspector, over the beard he grew.

One witness at the party where the carnage took place said Farook had suddenly left and returned a short while later heavily armed, dressed in black military-style gear and a mask -- and accompanied by his wife.

A home-made explosive that failed to detonate was later found at the scene of the shooting.

Reporters in apartment

The landlord of the couple's rented townhouse on Friday opened their home up to reporters and the public who flooded in, taking pictures and videos in a surreal scene.

Toys, a baby crib, a prayer rug and documents were scattered through the home where investigators found thousands of rounds of ammunition, as well as a makeshift bomb-making laboratory and 12 pipe bomb-like devices.

Journalists collect information inside the home of shooting suspect Syed Farook on December 4, 2015 in Redlands, California. Photo: AFP

Authorities identified the couple's victims as six women and eight men ranging in age from 26 to 60. All but two were county employees and colleagues of Farook.

Trudy Raymundo, the director of the San Bernardino health department, said she was getting coffee when the shooting started and she immediately hid under a table with a colleague.

Another woman who survived paid tribute to a colleague who shielded her when the shooting broke out and died.