The couple who massacred 14 people in California were discussing jihad and martyrdom online a year before they met in person, but there is no evidence Islamic militants arranged their marriage to facilitate an attack, the FBI says.
The FBI believes Syed Rizwan Farook, 28, the US-born son of Pakistani immigrants, and Tashfeen Malik, 29, a Pakistani native he married in Saudi Arabia last year, were each inspired by Islamic extremists before they became acquainted.
The precise origins of the couple's indoctrination in extremist ideology remain to be determined but appear to date back about two years, before Islamic State "became the global jihad leader that it is", FBI director James Comey said on Wednesday.
"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," the FBI director testified before the Senate judiciary committee.
The husband and wife were killed last week in a shootout with police hours after opening fire with assault rifles at a holiday gathering of Farook's co-workers at the Inland Regional Center social services agency in San Bernardino, about 100km east of Los Angeles.
Fourteen people were killed and 21 others were wounded in the December 2 assault, marking the deadliest burst of US gun violence in three years.
Comey said both the couple had declared that they were carrying out their attack "on behalf of" IS, an assertion he said they made "at or about the time" of the killings.
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.
In response to a question from Republican senator Lindsay Graham, Comey acknowledged that evidence of Islamic extremists playing such a role in their marriage would be "a game changer".
The investigation of Farook and Malik has also focused on his relationship with his boyhood friend Enrique Marquez, a Muslim convert who federal officials said made a 2011 or 2012 purchase of the two assault-style rifles used by the couple in the attack.
A government source familiar with the investigation said authorities were trying to determine if Farook had asked Marquez to buy the two rifles so as not to draw attention to himself.
CNN, citing unnamed US officials, reported Marquez has told investigators he and Farook had conceived an attack in 2012 in California, but they abruptly abandoned the plan, in part because they were spooked by unrelated FBI arrests around that time of four people charged with attempting to travel abroad to carry out jihad.
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.
Republican members of the Senate committee grilled Comey about the attack and criticised the Obama administration's response to the militant group IS.
The committee's chairman, Senator Charles Grassley, said the San Bernardino shootings had shown Obama to be "spectacularly wrong" about the security of the US visa screening process since Malik arrived in the US on a fiance visa on which he said she listed a false address.
Comey said in response to a question that he has no reason to believe IS 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".
