CEDAR RAPIDS (AP) -- Federal immigration agents on Monday arrested more than 300 people during a raid at a Postville meat processing plant.

The raid by agents from U.S. Immigration and Customs Enforcement was the largest in Iowa history, Matt M. Dummermuth, U.S. attorney for the Northern District of Iowa, said at a news conference.

Dummermuth said the raid at the Agriprocessors Inc. plant in northeast Iowa came after months of planning, beginning in October 2007. Federal agents were helped by state and local police.

Authorities said more than 300 workers were arrested and charged with immigration violations.

"Can't speculate on if that number will go up," said Claud Arnold, special agent in charge of ICE's Bloomington, Minn., office. "We'll have more information on that tomorrow."

Of those arrested, 44 were released for humanitarian reasons, primarily because they must care for children. Those released were ordered to report to court later.

The hundreds of people arrested were held in local jails or driven by bus about 75 miles to the National Cattle Congress fairgrounds in Waterloo. Federal authorities previously leased the fairgrounds and have turned it into a secure center.