The couple who carried out a massacre in San Bernardino, California last week had taken part in target practice days before the attack, in which 14 people were killed.
David Bowdich, assistant director of the FBI's Los Angeles office, told a news conference that Syed Farook and his wife, Tashfeen Malik, went to shooting ranges in the Los Angeles area.
He said the bureau believes both were radicalised and had been “for some time”, though he says the bureau does not know when or how that happened.
The FBI has said it is investigating the shooting at a holiday gathering of Farook’s co-workers as an act of terrorism.
19 pipes
Mr Bowdich said investigators had found 19 pipes in the couple’s home in Redlands, California, that could have been used to make bombs.
Earlier, the founder of an Islamic religious school organisation in Pakistan where Malik studied insisted the group is non-political and has no links to extremists.
Farhat Hashmi said Malik attended Al-Huda International Seminary's branch in the Pakistani city of Multan briefly between 2013 and 2014.
The school is for women only, with locations across Pakistan and in the US and Canada. Ms Hashmi said Malik left without completing a diploma.
She said the Al-Huda International Welfare Foundation denounces extremism, violence and acts of terrorism and cannot be held responsible “for personal acts of any of our students”.