In the hours before dawn one day in early October, Mehrdad Dalir found himself stranded at a bus station in Iran. He recalls feeling the heavy gaze of passers-by and shivering. He looked out of place, dressed in a prison outfit of sorts: gray sweatpants and sweatshirt, and a pair of blue plastic slippers.

About two days earlier, U.S. immigration authorities had taken Mr. Dalir from a detention facility inside an airport in Alexandria, La., and forced him aboard a plane to Iran as part of an unusual mass deportation to a country with a poor human rights record that the United States had bombed earlier this year in an effort to set back its nuclear program.

Mr. Dalir, 34, says he was shackled and handcuffed, and not given the chance to change out of the prison clothes he had worn during his time in Immigration and Customs Enforcement detention. Once in Iran, he made his way by metro and bus from Tehran’s international airport to his hometown, Mashhad, in northeast Iran.

After borrowing a cellphone, Mr. Dalir delivered the news to family members still in Mashhad: The United States had sent him back to Iran. “Maman, come and get me,” he told his mother, and broke into a sob. A blue backpack contained all his belongings — a few clothing items, his passport, immigration cards, a cellphone with a dead battery and a $20 bill.