Investigators were tracing the radicalisation of Syed Rizwan Farook, aged 28, and Tashfeen Malik, aged 29, who married in 2014.
Evidence mounted that both were interested in Islamist-inspired violence before they became acquainted.
“They were actually radicalised before they started... dating each other online, and as early as the end of 2013 they were talking to each other about jihad and martyrdom before they became engaged,” Comey said at a hearing of the senate judiciary committee.
A US government source said Farook may have been plotting an attack as early as 2011.
This diminishes the likelihood of early theories after the December 2 shootings in California that Malik had radicalised her husband. Twenty-one people were also wounded in the attack.
Classifying the massacre as a terrorist act, the Federal Bureau of Investigation believes that the couple, who were killed in a shootout with police a few hours after their attack on Farook’s San Bernardino County co-workers at the party, were inspired by foreign terrorist organisations.
Comey said it would be “very, very important to know” if their marriage had been arranged by a militant group as a way to carry out attacks in the US, although he said there was no evidence yet indicating that.
The investigation of Farook, who was born in Illinois to Pakistani immigrants, and Malik, who was born in Pakistan and lived most of her life in Saudi Arabia, is looking into his relationship with his boyhood friend Enrique Marquez.
According to media reports, Marquez had converted to Islam a few years ago and was connected to Farook’s family by marriage.
A government source said Farook may have planned an attack in 2011 or 2012 but did not follow through.
The source gave no details, but CNN cited two US officials as saying Farook and an unnamed second person selected a target but abandoned their plan after terror-related arrests in the area. Fox News reported Marquez told investigators about the aborted plot.
The San Bernardino Sun newspaper quoted investigation sources as saying multiple photographs of Carter High School in Rialto, California, were found on Farook’s phone.
The FBI said that in 2011 or 2012 Marquez legally bought the AR-15 assault-style rifles that Farook and Malik used in their attack on the San Bernardino party.
A government source said authorities were trying to determine if Farook had asked Marquez to buy the weapons so as not to draw attention to himself.
Marquez, who worked at a Walmart Supercenter in Corona, California, has not been arrested in the case, but he was questioned by the FBI on Tuesday and his family home was raided over the weekend. Marquez checked himself into a Los Angeles-area psychiatric facility soon after the shooting.
State documents showed Marquez married Mariya Chernykh last year, whose sister is married to Farook’s brother, Syed Raheel Farook, a US Navy veteran.
It could not be immediately determined if Marquez lived with his wife. The New York Times reported he split his time between his family’s home and that of a girlfriend.
On his marriage certificate, Marquez and his wife listed their religious society/ denomination as Islamic Society of Corona/Norco.
In a Facebook posting before the attack, Malik pledged loyalty to Islamic State, the militant group that has taken over parts of Iraq and Syria.