The husband and wife who carried out the California mass shooting showed signs of radicalisation but were not part of a broader network, the FBI says.

But Director James Comey noted there's still "a lot evidence that doesn't quite make sense".

Syed Farook and Tashfeen Malik didn't appear on the FBI's "radar screen" before the shooting on Wednesday that killed 14 in San Bernardino, California, said Mr Comey.

An IS-affiliated news agency Aamaq says the couple were "supporters" of the Islamic State group, but it stopped short of claiming responsibility for the attack.

The couple opened fire at a holiday banquet for Farook's co-workers before dying in a gunbattle with police.

The FBI is treating the attack as an "act of terrorism".

The couple attempted to destroy evidence, including crushing two mobile phones and discarding them in a rubbish bin, said David Bowdich, assistant director of the FBI's Los Angeles office.

Malik praised the leader of the IS group in a Facebook post just minutes into the attack, a Facebook executive said.