At least 300 people had been taken into custody at the Agriprocessors Inc. meat processing facility at Postville, with the total expected to reach as high as 700, The Des Moines Register reported. The plant is the largest kosher meatpacker in the United States, the newspaper said.

Tim Counts, a spokesman for U.S. Immigration and Customs Enforcement, said the agents conducted the raid under the auspices of a criminal search warrant allowing them to look for evidence related to aggravated identity theft fraudulent use of Social Security numbers and other illegal acts. A civil search warrant also allowed the agents to look for illegal immigrants, he said.

The newspaper reported the staff of Rep. Bruce Braley, D-Iowa, was told 600-700 arrests were likely. The plant employs about 1,000 workers.

"Based on information thus far developed in the investigation, it appears, based on 2007 fourth-quarter payroll reports, that approximately 76 percent of the 968 employees of Agriprocessors were using false or fraudulent Social Security numbers in connection with their employment," a federal affidavit filed in the case said.