The convicted Army major on death row for killing 13 people at Fort Hood has written a letter to the leader of ISIS, asking to become a "citizen" of the Islamic State's caliphate.

Army psychiatrist Nidal Hasan, 43, was sentenced to death last year by a military jury for shooting 13 people and injuring 32 others at Fort Hood in 2009.

However, on Thursday his lawyer confirmed that he has a handwritten letter addressed to infamous ISIS leader Abu Bakr al-Baghdadi.

"The letter states that Nadal Hasan wants to become a citizen of the Islamic State caliphate," said attorney John Galligan to CNN. "He wrote it in the last few weeks."

In the two-page letter, obtained by Fox News, Hasan states: "I formally and humbly request to be made a citizen of the Islamic State."

"It would be an honor for any believers 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," he continues.

The American-born Muslim signed the letter with "SoA," which stands for "Soldier of Allah."

According to Galligan, the letter "underscores how much of his life, actions and mental thought process are driven by religious zeal. And it also reinforces my belief that the military judge committed reversible error by prohibiting Major Hasan from both testifying and arguing ... how his religious beliefs" motivated him to kill American soliders.

Hasan is currently being held at the U.S. Disciplinary Barracks at Fort Leavenworth, Kansas.

Authorities said the Virginia native opened fire in November 2009 because he did not want to be deployed to Afghanistan and fight against other Muslims.

During the shooting, he targeted unarmed soldiers who were preparing to depart to Afghanistan or just returning, before he was eventually shot by two police officers. According to CNN, he was left paralyzed from his gunshot wounds.