The Federal Bureau of Investigation's terror probe has determined that US-born Syed Farook and Tashfeen Malik, a Pakistani national, were inspired by foreign terror groups and discussed "jihad and martyrdom" as early as 2013, Comey said.

The investigation "indicates that they were actually radicalised before they started courting or dating each other online," Comey told the Senate Judiciary Committee during a hearing.

The Islamic State group hailed the perpetrators and called them soldiers of the caliphate.

The assailants are accused of killing 14 people and wounding another 21 at a holiday party at a non-profit social services center in San Bernardino.

Farook and Malik traveled to the United States in July 2014, Farook with his US passport and Malik with her fiancee visa inside her Pakistani passport, the FBI has said.

Authorities said the couple had amassed a massive arsenal of weapons in their California home including thousands of rounds of ammunition, 12 pipe bombs, and bomb-making material.

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