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.

Comey, testifying at a hearing of the Senate Judiciary Committee, said there was yet no evidence that the marriage of Farook, who was born in the 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," Comey said.

The Federal Bureau of Investigation believes that 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. 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 United States.

The investigation of the San Bernardino, California, shooting also is 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 wounded 21 people. A government source familiar with the probe said investigators were trying to determine if Farook had asked Marquez the 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 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 taken over parts of Iraq and Syria. Coupled with Islamic State attacks in Paris in November that killed 130 people, the San Bernardino assault has elevated concerns about security and immigration in the United States.

Republican committee members grilled Comey about the attack and criticized the Obama administration's response to Islamic State. If the San Bernardino shooters are proven to have been inspired by Islamic militants, theirs would be the largest such attack since the Sept. 11, 2001, plots against the United States.

The committee's chairman, Republican Senator Charles Grassley, said the San Bernardino shootings had shown Obama to be "spectacularly wrong" about the security of the U.S. visa screening process since Malik arrived in the United States on a fiancée visa.

"Our government apparently didn't catch the false address in Pakistan that she listed on her application," Grassley said.

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," Comey said.

He said the latter scenario "has not been seen yet."-Reuters