SAN BERNARDINO, United States (AFP) - The mass shooting in California is being probed as "an act of terrorism," the FBI said late Friday as reports surfaced that the female assailant had pledged allegiance to the Islamic State group on Facebook.
"The investigation so far has developed indications of radicalization by the killers, and of potential inspiration by foreign terrorist organizations," FBI director James Comey told reporters.
But he said there was no indication that Syed Farook, 28, and his 29-year-old Pakistani wife Tashfeen Malik were part of any network. Authorities earlier had put her age at 27.
The developments come two days after the couple massacred 14 people and wounded 21 others 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 Farook and 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 said 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.
The massacre, if proven to be terror-related, would be the deadliest such assault on American soil since the September 11, 2001 attacks.
"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-ISIS 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.
Relatives were at a loss to explain how the couple, who had an infant daughter 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 become radicalised.
Attorney Mohammad Abuershaid said few people came in contact with Malik, who wore the full-face veil and was soft-spoken and shy.
"The women (in the family) communicated with her. Farook didn't want anyone else to talk to her," he said, adding that the men in the family had never even seen Malik's face.
Abuershaid said Farook had met Malik in 2013 through an online marriage site and had traveled to marry her in Saudi Arabia last year, where she lived.
One of Farook's colleagues said he was convinced Malik had radicalised her husband.
In Pakistan, Khalid Janbaz, former dean of the pharmacy department at Bahauddin Zakariya University, described her as a "brilliant student." And former classmate Abdia Rani told AFP she had "gradually turned religious" and over time, becoming more serious and withdrawn.
"But we never thought that she had extremist links or even can be an extremist," Rani said.
A US defense official, meanwhile, confirmed that Farook's brother was a decorated Navy veteran who won medals for his service during America's "war on terror." Abuershaid and fellow attorney David Chesley said Farook had apparently been teased by colleagues at the county health department, where he worked as an inspector, over the beard he grew.
One witness at the party where the rampage 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.
The landlord of the couple's rented townhouse opened their home up to reporters and the public who flooded in, taking pictures and videos in a surreal scene.
Toys, a 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 bombs.
Authorities identified the victims as six women and eight men aged 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 immediately hid under a table.
"It seemed like it went on forever," she told CNN.
"And all I could think of is why doesn't he stop? Why, why, why does he keep shooting?"