One of the San Bernardino shooters who killed 14 people pledged allegiance to ISIS on Facebook

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The revelation comes after US officials said the female shooter who carried out the mass shooting with her husband pledged allegiance to Islamic State (ISIS).

If a link to the terror group is confirmed, the mass shooting would be the first attack in America that has been led or inspired by ISIS.

An ISIS radio broadcast today confirmed that the two attackers were followers of the group.

Tashfeen Malik, who burst into a staff Christmas party with Syed Rizwan Farook, wielding assault rifles, pledged allegiance to ISIS leader Abu Bakr al-Baghdadi on Facebook, according to officials investigating the attack in San Bernardino, California.

The motive behind their deadly killing spree has still not been confirmed while investigators work to determining whether the couple's act was terrorism-related.

The post on social media by Malik on Wednesday was done on an account with a different name, according to one US official - but investigators did not explain how they knew she posted it.

The status on Facebook was posted just minutes before the deadly attack was carried out at the San Bernardino County Health Public Health Department.

The FBI said they are now investigating the attack as an act of terrorism.