A federal immigration raid at a kosher meatpacking plant in northeast Iowa was the largest such operation in U.S. history, with nearly 400 people arrested, federal officials said Tuesday.

Immigration and Customs Enforcement officials said at least 390 people were arrested on immigration charges as part of a raid Monday morning at Agriprocessors Inc. in Postville. The plant had about 900 workers before the raid.

Most of the 314 men and 76 women arrested are from Guatemala and Mexico, but some were from Ukraine and Israel.

"Based on the number of ... arrests, this is the largest single site operation of its kind ever in the United States," said ICE spokesman Tim Counts.

The raid followed a months-long investigation into Agriprocessors, the largest kosher slaughterhouse in the world.

ICE officials would not comment on whether company officials could face criminal charges. Telephone messages left with Agriprocessors on Tuesday were not returned.

Of those initially arrested, officials said 56 were released on humanitarian grounds, typically because their arrest would leave a child with no custodian. A handful were released because of medical conditions.

Men were being held at temporary housing at the National Cattle Congress Fairgrounds in Waterloo, where they are expected to be processed by Wednesday night and moved to other locations by Thursday. Women are being held at the Hardin County jail.

The American Civil Liberties Union of Iowa condemned the raids in a statement Tuesday.

"It appears that detainees are not receiving adequate time to meet with their lawyers, and that defense attorneys are being overwhelmed by requests to represent far more clients than is advisable or perhaps even ethical," said Ben Stone, the group's president. "We are concerned that the sheer size of this raid is likely to result in numerous violations of the U.S. Constitution, which protects the due process rights of all persons in this country."

Counts said those arrested had and would have adequate time to meet with their attorneys.

Everyone arrested Monday has been charged with immigration violations. So far 20 of them also have been arrested on a variety of criminal charges, including aggravated identity theft and false use of Social Security numbers, said Bob Teig, a spokesman for the U.S. attorney's office.

Those facing criminal charges began appearing Tuesday afternoon in a makeshift federal court at the Cattle Congress grounds in Waterloo, said U.S. Attorney Matt M. Dummermuth.

Anyone detained on a criminal charge will be placed in the custody of the U.S. Marshals Service. People charged only with immigration violations will remain in ICE custody before going through removal proceedings, including a hearing before an immigration judge. Those hearings will take place throughout the country, Counts said, depending on space.

According to a search warrant application and affadavit dated May 9, federal officials relied on a variety of sources, including former employees and at least one undercover source who wore a wire and became an employee of the plant at ICE's request.

According to the search warrant application, the undercover source said that some employees were paid with cash, and that those workers wore specially colored work hats to denote their status. Other employees were paid with checks that did not bear Agriprocessors' name.

Sources in the warrant application also said they saw what appeared to be underage workers at the plant.

According to the company's Web site, Agriprocessors was founded in 1987 by the Rubashkim family. The company's kosher and non-kosher products are found in many national supermarket chains.

Copyright 2008 Associated Press