“The investigation so far has developed indications of radicalization by the killers, and of potential inspiration by foreign terrorist organizations,” FBI director James Comey told reporters.
But he said there was no indication that Syed Farook, 28, and his 29-year-old Pakistani wife Tashfeen Malik were part of any network. Authorities earlier had put her age at 27.
The developments come two days after the couple massacred 14 people and wounded 21 others at a year-end office party in San Bernardino — the deadliest mass shooting in the United States since the Newtown school massacre in 2012.
US-born Farook and Malik were killed in a firefight with police hours after the attack, leaving investigators to comb through their belongings to try to determine a motive.
“We have uncovered evidence that has led us to learn of extensive planning,” David Bowdich, the assistant FBI director in charge of the Los Angeles office, told reporters.
“There’s a number of pieces of evidence that has essentially pushed us off the cliff to say we are now investigating this as an act of terrorism.”
He said investigators were examining a Facebook posting in which Malik is believed to have pledged allegiance to IS leader Abu Bakr al-Baghdadi, made around the time of Wednesday’s attack.
The massacre, if proven to be terror-related, would be the deadliest such assault on American soil since the September 11, 2001 attacks.
“They attempted to destroy their digital fingerprints and we found cell phones that were actually crushed in a nearby trash can,” Bowdich said.