James Comey said Tashfeen Malik and husband Syed Farook spoke of jihad and martyrdom during conversations on an online dating service in late 2013, BBC reported.
The FBI believes the duo were inspired by foreign terrorist organisations, but said the investigation was ongoing.
Last week's massacre was the deadliest terrorist attack in the US since 9/11.
The couple were both killed in a shoot-out with police hours after the attack.
Speaking during a Senate hearing in Washington, Comey characterised the couple as "homegrown violent extremists," and said that precise nature of the foreign influence was still under investigation.
Earlier this week, another FBI official said that there was no evidence uncovered so far to suggest the attack was plotted overseas.
Comey said the investigation "indicates that they were actually radicalised before they started courting or dating each other online," and noted discussions they had about jihad before their engagement last year. Malik moved to the US in July 2014 on a visa designed for fiances, and married Farook - US native - a month later.
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