The FBI has said it is now investigating the San Bernardino shooting in the US state of California as a "terrorist" attack, adding that it had uncovered new evidence. 

David Bowdich, assistant director of the FBI's Los Angeles office, also said the attackers tried to destroy evidence - crushing two cell phones and throwing them in a rubbish bin.

Do not let this cause mass hysteria; We're not there; We're not there at all.

David Bowdich, assistant director of the FBI's Los Angeles office

"Based on the information and the facts as we know them, we are now investigating these horrific acts as an act of terrorism," he told reporters of the attack that killed 14 people and wounded 21.

Bowdich said authorities were still trying to understand the attackers' motivations and whether they had been planning more attacks.

Ahead of the press conference, US media reported that the female attacker pledged allegiance to the Islamic State of Iraq and the Levant (ISIL) and its leader on Facebook and then deleted the post on the day of the incident.

Both the AP news agency and the CNN news network reported information about the online activities of Tashfeen Malik on Friday, citing officials.

If confirmed, the revelations could provide the first significant evidence of a motive for the shootings.

AP reported that a Facebook executive said Malik praised the leader of ISIL in a post.

The details of the Facebook post were not disclosed to the media outlets.


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Shoot-out deaths

The FBI's Bowdich said that he was aware of reports of a Facebook post but could not confirm the attackers had any ties to ISIL or were ISIL sympathisers. 

"We do not know that but we are looking into that," he said.

"Do not let this cause mass hysteria; We're not there; We're not there at all."

The Aamaq news agency, a website which relays ISIL news, said that followers of the group had carried out the attack on a social services agency in California.

Malik was a Pakistani woman who arrived in the US in 2014 on a fiancee visa before marrying Syed Rizwan Farook, who is suspected of having carried out the attack alongside her, in California.

The couple allegedly stormed a party attended by Farook's San Bernardino County co-workers in what appeared to be a planned attack, before dying in a shoot-out with police.

Source: Agencies