Fox said it obtained a copy of Hasan's letter to Abu Bakr al-Baghdadi from unnamed sources.
"I formally and humbly request to be made a citizen of the Islamic State," Hasan said in the handwritten letter. "It would be an honor for any believer to be an obedient citizen soldier to a people and its leader who don't compromise the religion of All-Mighty Allah to get along with the disbelievers."
Hasan, 43, was sentenced to death for a shooting spree at Fort Hood in 2009 that killed 13 people and wounded more than 30. The Virginia-born son of Palestinian immigrants, Hasan spent many years in the U.S. military, enlisting after he graduated from high school and rising to the rank of major after he obtained his medical degree.
IS, also known as ISIS (the Islamic State in Syria) or ISIL (the Islamic State in the Levant), began as an affiliate of al-Qaida in Iraq. The two groups are now enemies.