The FBI has declared the California massacre which killed 14 people was “an act of terrorism” but will not confirm widespread US media reports the female suspect had pledged allegiance to Islamic State (IS).

US media outlets are reporting that Tashfeen Malik, 27, posted a private message to herself on Facebook pledging her allegiance to the leader of the IS jihadist group Abu Bakr Al-Baghdadi.

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The post was later removed — it is not clear if the posting was made before or after the attack.

“We are looking into it,” the assistant director of the FBI’s LA office David Bowdich said.

Syed Rizwan Farook and his wife, both now dead, are the chief suspects. Photo: DMV

“Based on the information and the facts as we know them, we are now investigating these horrific acts as an act of terrorism.”

Malik and her husband, Syed Rizwan Farook, 28, were killed in a shootout with police hours after the Wednesday massacre during a holiday party at the Inland Regional Center social services agency in San Bernardino, about 100 kilometres east of Los Angeles.

Mr Bowdich said he was “not surprised” by reports the Aamaq news agency, which supports IS, said on its website that followers of the Islamist militant group had carried out Wednesday’s attack, saying the group was known to try to “attach themselves” to acts of violence.

Mr Bowdich also said the FBI was examining crushed mobile phones found near the shooting scene and had established that there were “telephonic connections” between the couple and other people of interest in FBI probes.

“We have uncovered evidence that has led us to learn of extensive planning,” Mr Bowdich said, confirming “some of the digital media was damaged”.

Investigation extends to suspect’s home country

Malik was a Pakistani citizen who had also lived in Saudi Arabia.

Pakistani intelligence has been assisting with the investigation, her family said.

The couple’s stash of firearms.

“I only found out about this tragedy today when some intelligence officials contacted me to ask me about my links with Tashfeen,” Malik’s uncle Javed Rabbani said.

“I had heard in the news that this tragedy had taken place but I could never even imagine that it would be someone from my family. Of course, we are in shock.”

He said his brother, Malik’s father, had become considerably more conservative since moving with his family to Saudi Arabia about 25 years ago.

Malik had not come to the attention of authorities while living in Saudi Arabia, according to a source close to the Saudi government.

She had moved back to Pakistan five or six years ago to study pharmacy, Pakistani officials said.