Asked about Senator Duckworth’s letter, the Department of Homeland Security said Monday that its Citizenship and Immigration Services division had denied Mr. Perez-Montes’s request for naturalization on March 15 because of the felony conviction.

“After his two tours of duty with the special forces, he came back a broken man due to the horrors he witnessed in Afghanistan and the physical brain injury he suffered while there,” said Christopher Bergin, Mr. Perez-Montes’s lawyer.

He said Mr. Perez-Montes’s role was to repair vehicles in Kandahar, and that his brain injury occurred after a grenade went off near his vehicle.

Mr. Perez-Montes’s family was not alerted before he was deported, Mr. Bergin said.

“His family was never able to hand him some money and some clothes before they deported him in his prison clothes,” Mr. Bergin said. “He had nothing else.”

Mr. Perez-Montes lived in Mexico until he was 8, when he came to the United States on a petition through a family member. He was raised in Chicago and has been a permanent legal resident since age 11, according to a statement on Senator Duckworth’s website. He was never in the country illegally, Mr. Bergin said.

Mr. Bergin said that Mr. Perez-Montes, who has two children who are citizens, was afraid that if he returned to Mexico he would become a target of cartels that would try to recruit him because of his military experience, or kill him if he refused. Mr. Bergin said last week that he planned to appeal the denial of citizenship “as far up the court ladder as we must climb.”