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The husband and wife suspected of carrying out a fatal attack on the Inland Regional Centre in San Bernardino on 3 December did not post jihadist messages on social media, according to FBI director James Comey.

The FBI's statement comes two weeks after the US media first reported that Syed Farook and Tashfeen Malik, suspected of killing 14 people and wounding 21 others, had posted messages of allegiance to the Islamic State group on social media shortly before or during the attack.

"So far, in this investigation we have found no evidence of posting on social media by either of them at that period in time and thereafter reflecting their commitment to jihad or to martyrdom," said Comey in a news conference on Wednesday.

Investigators still do not know why the couple chose to attack the Inland Regional Centre

"I've seen some reporting on that and that's a garble. The investigation continues, but we have not found that kind of thing," added the FBI director.

Comey also said that there was no evidence that the couple had direct contact with a terrorist organisation or were part of a terrorist cell.

Investigators still do not know why the couple chose to attack the Inland Regional Centre, where Farooq's work colleagues were having a gathering, or whether anyone else was involved in the attack, said NBC News.

The attacks - swiftly and widely attributed in US media to Islamic extremism - have led to an anti-Muslim backlash across the US and an increase in anti-Muslim hate crimes.

During the Republican presidential debate on Tuesday evening, candidates mentioned the San Bernardino attacks - that they attributed to "radical Islamist jihadists" - no fewer than 19 times.

The FBI director said that in online communications in 2013, the couple, who had then yet to meet in person, exhibited signs of their commitment to "jihad and martyrdom". Comey did not specify what those signs may have been.

"We can see from our investigation that in late 2013, before there is a physical meeting of these two people resulting in their engagement and then their journey to the United States, they're communicating online, showing signs in their communication of their joint commitment to jihad and to martyrdom," said Comey.