San Bernadino, California: On the day she and her husband killed 14 people and wounded 21 others at a holiday banquet in San Bernardino, California, a woman pledged allegiance to the Islamic State and its leader in a Facebook post using an alias, then deleted the messages before the attack, officials said Friday, as the F.B.I. announced that it was treating the massacre as an act of terrorism.

“The investigation so far has developed indications of radicalization by the killers, and of potential inspiration by foreign terrorist organizations,” the F.B.I. director, James B. Comey, said at a news conference. But he said that investigators had not found evidence that the killers were part of a larger group or terrorist cell. “There’s no indication that they are part of a network,” he said.

Both the U.S.-born husband and his spouse, Tashfeen Malik, 29, a native of Pakistan who lived in Saudi Arabia for more than 20 years, died in a shootout with police hours after Wednesday’s attack on a holiday party at the Inland Regional Center social services agency in San Bernardino, about 60 miles (100 km) east of Los Angeles.

“We have uncovered evidence that has led us to learn of extensive planning,” David Bowdich, the F.B.I. assistant director in charge of the Los Angeles office told reporters. At a news conference in San Bernardino, David Bowdich said he was aware of the post, which was taken down by Facebook on Wednesday, but he would not elaborate.

“There’s a number of pieces of evidence which has essentially pushed us off the cliff to say we are considering this an act of terrorism,” he said.

The remarkable disclosure about the online activities of Tashfeen Malik provided the first significant details suggesting a motive for her participation with her husband, Syed Farook, in the shooting.

Malik was a Pakistani woman who came to the U.S. in 2014 on a fiancee visa before Farook married her in California. They had a 6-month-old daughter. Malik’s alias on Facebook specifics about her postings were not publicly disclosed by the law enforcement official, who spoke on condition of anonymity because this person was not allowed to discuss an ongoing investigation.