The former Army psychiatrist who killed 13 people in the 2009 shooting spree of Fort Hood has written a letter expressing interest in becoming a citizen of the Islamic State, saying it would be an "an honor" to join the caliphate, his attorney told ABC News Thursday. It is unclear at what point the letter was intercepted after it was sent to ISIS.

Nidal Hasan, a former major, wrote a two-page letter to Abu Bakr al-Baghdadi, the notorious leader of ISIS, requesting to be a "citizen soldier" of the caliphate, which was first reported by Fox News. "The letter states that Nadal Hasan wants to become a citizen of the Islamic State caliphate," attorney John Galligan said. "He wrote it in the last few weeks."

On November 5, 2009, the 43-year-old opened fire on his fellow servicemen and women, killing 13 and injuring 32 others, UK MailOnline reported. Shot and paralyzed from the waist down in the resulting gunfire, he was sentenced to death last year and is on the military's death row at the U.S. Disciplinary Barracks at Fort Leavenworth, Kan.

Earlier this year, FBI Director James Comey testified that he believed the shooting carried out by Hasan was inspired by Al Qaeda.

Hasan, a Muslim American whose Palestinian immigrant parents raised him in Virginia, has described himself as a "mujahedeen," or Muslim holy warrior, in carrying out his attack. "I formally and humbly request to be made a citizen of the Islamic State," Hasan wrote in the letter, according to Fox News.

"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."

Hasan targeted unarmed soldiers who were preparing to deploy to Afghanistan or just returning, according to CNN. It is believed that he carried out the attack because he did not want to deploy to Afghanistan to fight other Muslims.

Although Hasan offered no defense during his trial, he told the judge there was "adequate provocation" for the attack since the soldiers were going to participate in "an illegal war" in Afghanistan, additionally describing himself to be a soldier who "switched sides."

Meanwhile, news of Hasan's letter comes days after Minnesota native Douglas McAuthur McCain, 33, was said to have died while fighting with the brutal Al Qaeda breakaway organization.