Up to 700 employees at the largest kosher meatpacking plant in the world could be arrested after an immigration raid on Monday. Federal agents descended on the AgriProcessor plan in Postville, northeast Iowa, around 10 a.m. to execute a criminal search warrant for evidence relating to identity theft, fraudulent use of Social Security numbers, and other crimes, according to US Immigration and Customs Enforcement. Agents were also executing a civil search warrant for people residing illegally in the United States. "This is not the first time this office has been a part of a criminal worksite enforcement operation," said Matt Dummermuth, the United States attorney for the Northern District of Iowa. "However, this is the largest operation of its type ever in Iowa." Around 500 agents and officers from federal, state and local agencies were involved in the raid, which had been in the works for months. Anyone encountered during the operation who is discovered to be in the US illegally will eventually be placed in deportation proceedings, officials said a press conference on Monday afternoon. By midday Immigration and Customs Enforcement agents arrested more than 300 people for immigration violations, and they were being interviewed by Enforcement agents and Public Health Service officers to determine if they have health, caregiver or other humanitarian concerns. As a result, more than 40 people have so far been released on humanitarian grounds pending immigration proceedings. This incident is the latest to befall meat processing plants owned by the Rubashkin family in the last few years. The plant in Postville, which opened in 1987, has been the target of several campaigns alleging inhumane slaughter practices and labor rights violations. Last month, the slaughterhouse was fined $182,000 by the Iowa Division of Labor for a broad range of health and safety violations, including failure to provide workers with proper safety training and to label toxic chemicals. AgriProcessor's Postville plant has been under scrutiny for its labor record since 2006, when the Forward newspaper reported on working conditions in a series of articles. After learning of the raid, Congressman Bruce Baley of Iowa said he hoped there would be consequences for the employers, not just the employees. "Until we enforce our immigration laws equally against both employers and employees who break the law, we will continue to have a problem with immigration," Baley said.