On the 42nd day of his confinement at an immigration detention center in the pine woods of central Louisiana, Alireza Doroudi seemed to be on the verge of getting out.

A few minutes before his latest immigration hearing last Thursday, the government said it was dropping one of the charges against him. Other international students who, like him, had been detained by the authorities were being released, one by one.

But shortly before the hearing began, Mr. Doroudi’s fiancée, Sama Ebrahimi Bajgani, one of the few people who had come to watch, was not hopeful. “I don’t think I can trust anything, to be honest,” she whispered.

A native of Iran, Mr. Doroudi, 32, had been in the country doing doctoral work in mechanical engineering at the University of Alabama. He had no run-ins with the law beyond a couple of traffic citations.

But early in the morning on March 25, U.S. immigration agents showed up at Mr. Doroudi’s apartment in Tuscaloosa and took him into custody.

At the time of the arrest, authorities told the news media that Mr. Doroudi posed “significant national security concerns.” They never elaborated, even in court, Mr. Doroudi’s lawyer said. The charges against him only concerned his legal status as an international student.