Cedar Rapids, Ia. - The number of illegal immigrants detained Monday in Postville has risen to 390 in what federal officials now describe as the largest single-site raid of its kind nationwide. The detainees include 314 men and 76 women, according to figures released this morning by federal authorities. Fifty-six detainees - mostly women with young children - have been released under the supervision of U.S. Immigration and Customs Enforcement.

"We're here to discuss not only the largest operation of its kind ever in Iowa, but in fact the largest single-site enforcement operation of its kind in the country," U.S. Attorney Matt M. Dummermuth said.

The detainees included 290 who claimed to be Guatemalans, 93 Mexicans, three Israelis and four Ukrainians. Among the detained were 12 juveniles, six of whom have been released.

Customs and law enforcement agents worked through the night processing the detainees, said Claude Arnold, the ICE special agent in charge of the operation. Detainees were "administratively arrested" but have not yet been criminally charged, he said.

Detainees who are charged with aggravated identity theft, unlawful use of a Social Security number or other offenses will be given lawyers and sent to appearances in one of three makeshift courtrooms at the detainee center in Waterloo, Arnold said.

The set-up includes three courtrooms - two in trailers and one in an existing room. A spokesman for the U.S. Attorney's office said the court proceedings would be open within space constraints.

Video footage shot by federal agents showed a large "intake area" in McElroy Auditorium with folding chairs and tables. The footage, which did not show any detainees, included images of a kitchen, break room, restroom and shower facilities used by detainees.

Arnold would not disclose how many people were involved in Monday's effort, citing security concerns.

Two injuries were reported last night at the detention center in Waterloo, Arnold said. One involved a man who fled from agents and injured his leg. The man was treated at a hospital and released.

A second injury involved a customs agent who suffered minor cuts after slipping and falling down a stairwell.

"The plans went very well" so far, Arnold said. "This is a huge undertaking."

Dummermuth declined to comment about possible charges against managers at Agriprocessors, Inc., citing the ongoing investigation. A federal affidavit released Monday detailed several eyewitness accounts of employee abuse, including one floor manager who allegedly struck a worker with a meat hook.

Nationwide, ICE agents arrested 863 people on criminal charges in 2007 and made more than 4,000 administrative arrests - a tenfold increase from five years before, according to the agency's Web site.

A similar factory raid in New Bedford, Massachusetts last year netted 361 arrests, most of whom faced deportation and were separated from their families.

Arnold said authorities were providing the Waterloo detainees with three meals a day and a nightly snack. The women detained were sent to the Hardin County Jail in Eldora. Arnold said that sheriff's deputies at the jail would turn away any media representatives or visitors trying to contact the inmates.

A second press conference is planned for 4 p.m. today at the U.S. Attorney's office in Cedar Rapids.