POSTVILLE, Iowa  When federal immigration agents raided the kosher meatpacking plant here in May and rounded up 389 illegal immigrants, they found more than 20 under-age workers, some as young as 13.

Now those young immigrants have begun to tell investigators about their jobs. Some said they worked shifts of 12 hours or more, wielding razor-edged knives and saws to slice freshly killed beef. Some worked through the night, sometimes six nights a week.

One, a Guatemalan named Elmer L. who said he was 16 when he started working on the plant’s killing floors, said he worked 17-hour shifts, six days a week. In an affidavit, he said he was constantly tired and did not have time to do anything but work and sleep. “I was very sad,” he said, “and I felt like I was a slave.”

At first, labor officials said the raid had disrupted federal and state investigations already under way at Agriprocessors Inc., the nation’s largest kosher plant. The raid has drawn criticism for what some see as harsh tactics against the immigrants, with little action taken against their employers.