A couple who massacred 14 people at a California holiday party were discussing martyrdom online before they met in person and married, FBI director James Comey said on Wednesday.
A US government source familiar with the investigation of the shooting said Syed Rizwan Farook (28), may have been plotting an attack on a US target as early as 2011.
Mr Comey, testifying at a hearing of the Senate Judiciary Committee, said there was 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 a militant group.
“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,” Mr Comey said.
The FBI believes the couple, who were killed in a shootout with police a few hours after their attack on the party, were inspired by foreign terrorist organisations.
Mr Comey said it would be “very, very important to know” if their marriage last year had been arranged as a way to carry out attacks in the US.
The investigation of the San Bernardino, California, shooting is also looking at the relationship between Farook and boyhood friend Enrique Marquez, who reportedly had converted to Islam a few years ago and was connected to Farook's family by marriage.
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 party, which also wounded 21 people.
A government source familiar with the inquiry said investigators were trying to determine whether Farook had asked Marquez to buy the weapons so as not to draw attention to himself.
Marquez, who worked at a Walmart 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 that last year Marquez married Mariya Chernykh, 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 that he split his time between his family’s home and that of a girlfriend.
Gasser Shehati, a friend of Farook’s from a San Bernardino mosque, said Farook told him several years ago that Marquez had converted to Islam.
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 had pledged loyalty to Islamic State, the militant group that has taken over parts of Iraq and Syria.
Coupled with Islamic State-linked attacks in Paris in November that killed 130 people, the San Bernardino assault has elevated concerns about security and immigration in the US.
Republican committee members grilled Mr Comey about the attack and criticised the Obama administration’s response to Islamic State.
If the San Bernardino shooters are finally proven to have been inspired by Islamic militants, theirs would be the largest such attack since against the US since the September 11th, 2001, plots.
The committee's chairman, Republican Senator Charles Grassley, said the San Bernardino shootings had shown Mr Obama to be "spectacularly wrong" about the security of the US visa screening process, since Malik had arrived in the country on a fiancee visa.
“Our government apparently didn’t catch the false address in Pakistan that she listed on her application,” Mr Grassley said.
Mr Comey said in response to a question that he has no reason to believe Islamic State already has cells in the US.
"They are trying to motivate people already in the United States to become killers on their behalf and they would very much like to - as they aspire to be the leader in the global jihad - send people here to conduct attacks," Mr Comey said.
He said the latter scenario “has not been seen yet”.