FBI, left, and California Department of Motor Vehicles via AP
This undated combination of photos provided by the FBI, left, and the California Department of Motor Vehicles shows Tashfeen Malik, left, and Syed Farook.
"San Bernardino involved two killers who were radicalized for quite a long time before their attack," Comey told the Senate Judiciary Committee. "They were actually radicalized before they started courting or dating each other online."
He added: "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 lived together in the United States."
Comey described Syed Rizwan Farook, 29, and his wife, Tashfeen Malik, 27 - who were married in 2014 - as "homegrown violent extremists" who were inspired by foreign terror organizations. He said the investigation is continuing.
"We're working very hard to understand exactly their association and the source of their inspiration," Comey said.
Investigators are looking into the possibility that Farook had been considering launching an attack as early as 2011, a US government source told Reuters.
Officials said in a press conference on Monday that Farook and Malik had been radicalized for "quite some time" and visited gun ranges for "target practice" in the days leading up to the assault on December 2, which left 14 people dead and injured at least 21 more at the Inland Regional Center in San Bernardino.
Jonathan Ernst/Reuters FBI Director James Comey
David Bowdich, assistant director of the FBI's Los Angeles office, said that while investigators had still not pinpointed a motive, they had reason to believe that both had been radicalized "for quite some time."
Malik, who came to the US with Farook from Pakistan on a K-1 visa - known as a "fiancé" visa - in 2014, reportedly pledged her allegiance to ISIS leader Abu Bakr al-Baghdadi on Facebook shortly after the attack began.
Malik obtained a green card, and thus permanent legal residence, in the summer of 2015, ABC reported.
Bowdich said that there is no evidence, so far, that the plot had been coordinated with anyone living outside the US, corroborating Comey's statements last Friday that "we have no indication so far that these killers are part of an organized larger group or form part of a cell."
Investigators are also looking into a $28,000 transfer made to the couple's bank account days before the attack.
Both died as they were being pursued by police on Wednesday, firing 76 rifle rounds before they were ultimately neutralized, San Bernardino Police Chief Jarrod Burguan said in a press conference last week.
When police searched the suspects' home, they found "several hundred" .22-caliber long-rifle rounds, "12 pipe bomb-type devices" in the garage, "hundreds of tools, many of which could be used to construct IEDs or pipe bombs," 2,000 9 mm rounds, and 2,500 .223-caliber rounds of ammunition.