Story highlights Nidal Hasan is on the military's death row at Fort Leavenworth

He wrote a letter to Abu Bakr al-Baghdadi, his attorney says

In the letter, Hasan says it would be an honor to be an "obedient citizen soldier"

The Army psychiatrist who killed 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, his attorney said Thursday.

"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."

Details of the letter were first reported by Fox News on Thursday.

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Described as a two page letter, it was addressed to Abu Bakr al-Baghdadi , the notorious leader of ISIS who declared himself the caliph -- the religious ruler -- over what he calls the Islamic State that he says stretches from western Syria to eastern Iraq.

Photos: Terror attacks on U.S. soil Photos: Terror attacks on U.S. soil Terror attacks on U.S. soil – A pair of bombs went off near the finish line of the packed Boston Marathon on April 15, 2013, leaving three people dead. More than 200 people were wounded. One suspect, Dzhokhar Tsarnaev, has been charged with 30 federal counts related to the attacks. His brother, Tamerlan, was killed during pursuit by police. Hide Caption 1 of 7 Photos: Terror attacks on U.S. soil In this handout photo provided by the U.S. Army, police take cover as Nidal Hasan opens fire in Fort Hood, Texas, on November 5, 2009. The Army psychiatrist was convicted of killing 13 people. Hide Caption 2 of 7 Photos: Terror attacks on U.S. soil A man stands near the rubble, asking if anyone needs help, after the collapse of one of the World Trade Center towers on September 11, 2011. In what was the worst terrorist attack in U.S. history, 2,753 people were killed when two hijacked planes were intentionally crashed in the north and south towers of the New York buildings. Two other planes were also hijacked: One crashed into the Pentagon in Washington, and one crashed at a field near Shanksville, Pennsylvania. Hide Caption 3 of 7 Photos: Terror attacks on U.S. soil Atlanta was the excited and elated host of the 1996 Summer Olympics when a bomb went off at Centennial Olympic Park on July 27. Two people were killed and 111 were injured by the blast. Eric Rudolph was convicted of placing the 40-pound bomb, which was filled with nails and screws. Hide Caption 4 of 7 Photos: Terror attacks on U.S. soil The north side of the Alfred P. Murrah Federal Building in Oklahoma City shows the devastation caused by a fuel-and-fertilizer truck bomb on April 19, 1995. At the time, it was the worst terror attack on U.S. soil, killing 168 people. Timothy McVeigh and Terry Nichols were convicted of the attack. Both were former U.S. Army soldiers associated with the militant Patriot Movement. Hide Caption 5 of 7 Photos: Terror attacks on U.S. soil A police photographer helps document the bombing of the underground parking garage at the World Trade Center. That bombing killed six people on February 26, 1993. Six suspects were convicted of participating in the bombing. The seventh suspect, Abdul Rahman Yasin, is still at large. Ramzi Yousef directed the organization and execution of the bombing. He said he did it to avenge the sufferings Palestinian people had endured at the hands of U.S.-aided Israel. Hide Caption 6 of 7 Photos: Terror attacks on U.S. soil Three people were killed and 23 others were wounded after a string of mail bombings carried out by Ted Kaczynski, aka "The Unabomber," from 1978 to 1995. Here, FBI agents guard the entrance to Kaczynski's property in Lincoln, Montana, on April 5, 1996. In May 1998, Kaczynski received eight life sentences for his crimes. Hide Caption 7 of 7

"I formally and humbly request to be made a citizen of the Islamic State," Hasan wrote in the letter, according to Fox News.

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"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 is on the military's death row at the U.S. Disciplinary Barracks at Fort Leavenworth, Kansas.

Hasan was sentenced to death by a military court for the November 5, 2009, rampage, which authorities said the American-born Muslim carried out because he did not want to deploy to Afghanistan to fight other Muslims.

Hasan was an Army major when he carried out the rampage at a deployment center, targeting unarmed soldiers who were preparing to deploy to Afghanistan or just returning. Thirty-two people were also injured in the attack.

His rampage was brought to an end when he was shot by two police officers. The shooting left him paralyzed.