David Bowdich also announced that the bureau would be the lead agency investigating the deadly rampage. Evidence gathered indicated that the shootings were planned extensively, officials said during a press conference.
He also said the shooters attempted to destroy evidence, including crushing two cellphones and discarding them in a trash can.
Earlier that morning, a U.S. official said San Bernardino suspected shooter Tashfeen Malik pledged allegiance to the Islamic State group and the terror group's leader, Abu Bakr al-Baghdadi, on Facebook using an alias then deleted the messages before the attacks.
Malik's online activities provided the first significant details suggesting a motive for her participation with her husband, Syed Farook, in the shooting at the Inland Regional Center that left 14 dead and 21 others injured on Wednesday.
MORE: All fatal San Bernardino shooting victims identified
Farook, 28, was an American citizen, while Malik, 29, was in the United States on a Pakistani passport and had a K1 "Fiance" Visa, according to David Bowdich, assistant director of the Los Angeles FBI office. She later obtained a Green Card.
Farook traveled internationally and re-entered the U.S. with Malik in July 2014. The couple married in Riverside and had a 6-month-old daughter.
A U.S. official said Malik expressed "admiration" for the extremist group's leader on Facebook under the alias account and said there was no sign that anyone affiliated with the Islamic State group communicated back with her and no signs of any operational instructions being conveyed to her. This official spoke on condition of anonymity because this person was not authorized to discuss case details by name.
The San Bernardino rampage was the nation's deadliest mass shooting since the attack at Sandy Hook Elementary School in Newtown, Connecticut that left 26 children and adults dead in 2012.
The FBI does not have evidence yet that ISIS directed or financed this plot. So far this massacre can be categorized as at least in part inspired by ISIS, though workplace issues may have been involved as well.
Attorney General Loretta Lynch cautioned that it could have been work-related rage or a twisted hybrid of religion and personal vendetta.
MORE: What we know about San Bernardino mass shooters
Malik helped her husband kill 14 people at a holiday banquet for his county co-workers before dying in a gunbattle with police.
The suspects made sure to cover their tracks by smashing their cellphones and hard drives, law enforcement sources tell ABC News.
Digital media recovered at multiple scenes connected to the shooting have been sent to an FBI laboratory in Virginia, where computer forensics analysts will try to reconstruct and extract any digital information they can. The bureau hopes that some of the information, such as email and texting, can be retrieved from internet providers and mobile phone companies.
MORE: Redlands home connected to suspects packed with weapons
The couple had also stockpiled 12 pipe bombs, tools to make more explosives and well over 4,500 rounds of ammunition at their Redlands home.
A lawyer who's representing the Farook family is questioning the way authorities have described the attack and has also offered explanations for some of the accusations.
"When people have guns and they have ammo, a lot of times when they go into shooting and firing ranges they do waste a lot of ammo at these ranges. Having a good amount of ammo doesn't mean that you're planning an attack. They're making points about the SUV he was driving. He's rented SUVs before. This isn't the first time that he's done this. There wasn't anything to the family to show or signify that he's doing something out of the ordinary," attorney Mohammad Abuershaid said.
ABC News and The Associated Press contributed to this report.