Investigators say Tashfeen Malik, 27, left a post on a Facebook page using an alias pledging allegiance to ISIS and its leader al-Baghdadi.
ISIS-affiliated news agency Aamaq said Farook and Malik were 'supporters' of the terror group but stopped short of claiming responsibility for attack.
FBI is now investigating the San Bernardino mass shooting as 'an act of terror'.
Two other federal law enforcement officials told the newspaper on Friday that Malik also pledged allegiance to a leader of the ISIS (Daesh) terrorist group, also known as Daesh, which is outlawed in a range of countries including Russia.
Earlier on Friday, FBI Director James Comey told reporters that investigators have discovered "indications of radicalization by the killers and of potential inspiration by foreign terrorist organizations."
Comey added, however, that it was not clear whether the San Bernardino shooters were part of a terrorist organization or network.
"We have no indication that these killers are part of an organized larger group or form part of a cell. There is no indication that they are part of a network," the FBI Director said.
FBI Los Angeles Assistant Director David Bowdich said on Friday that the San Bernardino shooting was being probed as an act of terrorism, but could not confirm that the California gunmen were influence by the Daesh.
On Wednesday, a shooting at Inland Regional Center in San Bernardino, California (about 60 miles east of Los Angeles) took the lives of 14 people, injuring over 20 others.
Police identified the shooters as a US-born man of Pakistani origin Syed Rizwan Farook and his wife Tashfeen Malik, a Pakistani national who was in the United States on a K1 (fiancée) visa. The couple was killed in a shootout with police hours after the Wednesday attack; Sputnik reported.
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