The mass shooting in California is being investigated as "an act of terrorism," the FBI said Friday, 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. FBI director James Comey said the probe so far had shown that the couple were radicalised and inspired by "foreign terrorist organisations," but were not part of a larger group or cell. Farook had contact with people from at least two militant organizations overseas, including the al Qaeda-affiliated Nusra Front in Syria, the Los Angeles Times reported on Friday, 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. A pro-IS news agency, Aamaq, on Friday said the mass shooting was perpetrated by sympathisers of the radical group, which has urged followers in the United States and elsewhere to carry out lone wolf attacks. 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. Attorney Mohammad Abuershaid said few people came in contact with Tashfeen, who wore the full-face veil and was very soft-spoken.