The husband and wife, who killed 14 people in an attack on a party in San Bernardino, California, were radicalised before they started dating online, according to FBI Director James Comey.
We believe they [attackers] were inspired by foreign terror organisations.
The FBI probe has determined that US-born Syed Farook and Tashfeen Malik, a Pakistani national, were inspired by foreign armed groups and discussed "jihad and martyrdom" as early as 2013, Comey said on Wednesday while testifying at a hearing of the Senate Judiciary Committee.
The investigation "indicates that they were actually radicalised before they started courting or dating each other online", Comey said.
"And online, as early as the end of 2013, they were talking to each other about jihad and martyrdom before they became engaged and then married and lived together in the United States.
"We also believe they were inspired by foreign terror organisations," Comey said.
The assailants are accused of killing 14 people and wounding 21 at a holiday party at a non-profit social services centre in San Bernardino.
The Islamic State of Iraq and the Levant (ISIL) group hailed the perpetrators and called them soldiers of the caliphate.
'Armed groups not involved'
Comey added that there was yet no evidence that the marriage of Farook, who was born in Illinois to Pakistani immigrants, and Tashfeen Malik, 29, who was born in Pakistan and lived most of her life in Saudi Arabia, was arranged by an armed group.
The FBI, Comey added, is "working very hard to understand whether there was anybody else involved with assisting them, with supporting them, with equipping them. And we're working very, very hard to understand, did they have other plans".
Farook and Malik travelled to the US in July 2014, the FBI said, adding that Farook used his US passport and Malik used her fiancee visa inside her Pakistani passport in the process.
Malik is believed to have pledged allegiance to ISIL in a Facebook post made under another name, around the time of the attack, according to media reports. The post was reportedly deleted by Malik.
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.
Comey's revelations were part of a broader testimony about FBI oversight and what he called "a staggering array of threats that face our country - in terrorism, in counter-intelligence and in criminal matters".