Last year, at least 60,000 immigrants worked in the federal government’s nationwide patchwork of detention centers — more than worked for any other single employer in the country, according to data from United States Immigration and Customs Enforcement, known as ICE. The cheap labor, 13 cents an hour, saves the government and the private companies $40 million or more a year by allowing them to avoid paying outside contractors the $7.25 federal minimum wage. Some immigrants held at county jails work for free, or are paid with sodas or candy bars, while also providing services like meal preparation for other government institutions.

Unlike inmates convicted of crimes, who often participate in prison work programs and forfeit their rights to many wage protections, these immigrants are civil detainees placed in holding centers, most of them awaiting hearings to determine their legal status. Roughly half of the people who appear before immigration courts are ultimately permitted to stay in the United States — often because they were here legally, because they made a compelling humanitarian argument to a judge or because federal authorities decided not to pursue the case.

“I went from making $15 an hour as a chef to $1 a day in the kitchen in lockup,” said Pedro Guzmán, 34, who had worked for restaurants in California, Minnesota and North Carolina before he was picked up and held for about 19 months, mostly at Stewart Detention Center in Lumpkin, Ga. “And I was in the country legally.”

Mr. Guzmán said that he had been required to work even when he was running a fever, that guards had threatened him with solitary confinement if he was late for his 2 a.m. shift, and that his family had incurred more than $75,000 in debt from legal fees and lost income during his detention. A Guatemalan native, he was released in 2011 after the courts renewed his visa, which had mistakenly been revoked, in part because of a clerical error. He has since been granted permanent residency.

Claims of Exploitation

Officials at private prison companies declined to speak about their use of immigrant detainees, except to say that it was legal. Federal officials said the work helped with morale and discipline and cut expenses in a detention system that costs more than $2 billion a year.