By Veronique Dupont and Jocelyne Zablit
The husband and wife behind the shooting in California that left 14 people dead were radicalised for "quite some time" and both went for target practice before the massacre, authorities say.
The update from the FBI on the terror probe into the December 2 rampage in San Bernardino came after US President Barack Obama vowed to destroy the Islamic State group and hunt down its followers, in an address to a jittery nation.
The self-proclaimed Islamic State has praised the California attackers - Syed Farook and Tashfeen Malik - as "soldiers" of its self-proclaimed caliphate but did not say they were members of the group.
"As the investigation has progressed, we have learned and believe that both subjects were radicalised and have been for quite some time," David Bowdich, the FBI's assistant director in Los Angeles, told reporters on Monday.
"The question for us is how and by whom and where were they radicalised?" he said.
"Maybe there's not a by whom. Often times it's on the internet."
He said the couple, killed in a shootout with police following the gun attack on a year-end office party, went for target practice in ranges around the Los Angeles area, including in the days before the tragedy.
On Sunday (local time), Obama said the attack was an "act of terrorism," and the Federal Bureau of Investigation is investigating it as such.
Authorities have, however, said there is no indication the couple was part of an organised larger group or cell.
In addition to two rifles and two handguns used in the attack at the party organised by Farook's employer - the San Bernardino County health department - investigators found an impressive arsenal at their home.
"The threat from terrorism is real, but we will overcome it. We will destroy (IS) and any other organisation that tries to harm us," Obama said Sunday, in only his third address from the Oval Office since taking office in 2009.
Farook, an American citizen, and his Pakistani wife met on an online dating site. Last year, he travelled to Saudi Arabia, where she lived, and they married. They left behind a six-month-old daughter.
Fox News also reported a deposit of US$28,500 was made to Syed Farook's bank account two weeks before he and his wife went on the rampage.
Investigators are trying to determine whether the November 18 transaction, made from Utah-based WebBank.com, was a loan, Fox News says.