The students, who were earning about $8 an hour, said they were isolated within the plant, rarely finding moments to practice English or socialize with Americans. With little explanation or accounting, the sponsor took steep deductions from their paychecks for housing, transportation and insurance that left many of them too little money to afford the tourist wanderings they had eagerly anticipated.

Program documents and interviews with 15 students show that Cetusa failed to heed many distress signals from students over many months, and responded to some with threats of expulsion from the program.

A Cry for Help

Mr. Ureche, 22, an engineering student, said he had begun to appeal to Cetusa for a different job as soon as he went to work lifting boxes loaded with Hershey’s candies.

“I’ve been having serious back pains since the first day of work,” Mr. Ureche reported in his e-mail to the State Department on June 6, sent two weeks after he started on the job. “If I continue in this rythm of work, it may cause me serious health damages.”

He felt “mistreated and ignored by my sponsor,” he wrote. And the organization told him, he said, that if he complained to Washington, “they will immediately cancel my visa." A few days later Mr. Ureche quit his job, making his way to New York and finding work.

When the walkout came two months later, State Department officials reacted swiftly, opening an investigation centering on Cetusa that has not yet concluded.

The department was already on notice about trouble in the program, after an Associated Press investigation last year found abuses of foreign students in several states. Officials started a broad review earlier this year, and on July 15 they inaugurated new regulations, which tighten requirements on sponsor organizations to ensure that students are matched with jobs that are appropriate and safe. A newly expanded staff of 18 inspectors will begin on-site audits of sponsor organizations this fall, officials said.