Officials said that as the attack in San Bernardino was happening, 27-year-old female shooter Tashfeen Malik posted a pledge of allegiance to IS leader al-Baghdadi on Facebook, three US officials familiar with the probe said.

They said Malik's post was made on an account with a different name, one US official said. The officials did not explain how they knew Malik made the post.

Another official said authorities haven't ruled out that others may have influenced this radical view.

In addition, the law enforcement source said investigators have a greater focus on whether the shooting occurred after a workplace issue with religion.

Malik may have radicalised her husband investigators said a day after the heavily-armed couple opened indiscriminate fire during a party at a social service centre for people with disabilities in San Bernardino, California.

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