How they were radicalized is still under investigation, said David Bowdich, assistant director at the FBI's Los Angeles bureau.
Both shooters participated in some target practice, according to Bowdich.
Bowdich said officials "do not see any evidence of" which shooter -- Syed Farook or his wife, Tashfeen Malik -- "was in control" of the attack.
"And I want to be crystal clear here -- we do not see any evidence so far of a plot outside the continental U.S.," said Bowdich. "We may find it some day, we may not, we don't know. But right now we're looking at these two individuals and beginning to focus on building it out from there."
"We will get to to the bottom of this," he said. "We want to find out everyone who participated in the pre-planning, if there was anyone else." "We don't know everything yet," he said, adding, "but we will leave no stone unturned."
Five guns were recovered, said ATF Assistant Special Agent in Charge John D'Angelo, and all of them were purchased between 2007 and 2012. Farook purchased two pistols and one rifle, while his former neighbor, Enrique Marquez, purchased the two assault rifles, said D'Angelo.
"Right now, our major concern for the FBI is determining how those firearms and rifles in particular got from Marquez to Farook and Malik," said D'Angelo.
The FBI would not comment on Marquez's whereabouts.