JACKSON, Miss. (AP) — One of Mississippi's most successful private companies pleaded guilty Thursday to conspiracy to violate immigration laws related to the nation's largest workplace raid on illegal immigrants, federal authorities said.

Immigration agents detained more than 600 illegal immigrants at Howard Industries' electrical transformerplant in Laurel during a massive raid on Aug. 25, 2008. Most of them were deported, though a few were convicted on identity theft charges for using fraudulent documents and providing fake papers to other workers.

The company had repeatedly denied knowing that illegal immigrants worked at the plant, but pleaded guilty in U.S. District Court to one count of conspiracy. The company agreed to pay a $2.5 million fine, which is larger than normal for such a conviction, the U.S. Justice Department said in a news release.

The company said in a statement that it was pleased to resolve the investigation "based on the actions of its former human resources manager Jose Humberto Gonzalez."

Gonzalez is the only company executive who has been charged in the case. He pleaded guilty in December 2009 to conspiracy and admitted that he hired hundreds of people who he knew were in the country illegally.

Gonzalez's sentencing has been delayed several times without explanation while the investigation continued, leading legal experts to speculate that he's been cooperating with investigators. His attorney did not immediately respond Thursday to messages from The Associated Press.

Howard Industries said that the illegal workers used fraudulent documents to "circumvent the numerous identification checks" the company used. But prosecutors said the company knowingly employed illegal immigrants, and even hired some of them after the Social Security Administration told the company that their Social Security numbers were not valid. Gonzalez admitted to similar allegations when he pleaded guilty before the same judge.

Those detained in the raid came from countries including Brazil, El Salvador, Germany, Guatemala, Honduras, Mexico, Panama and Peru.

U.S. Attorney John M. Dowdy Jr. said Howard Industries' hiring practices took jobs away from American citizens who needed them and encouraged illegal immigration.

"The unlawful actions by companies like Howard Industries in this case contribute significantly to the problem of illegal immigration in the United States," he said in a statement.

Howard Industries makes dozens of products from electrical transformers to medical supplies. The plant that was raided is in Mississippi's Pine Belt region, known for commercial timber growth and chicken processingplants.

In 2002, Mississippi lawmakers approved a $31.5 million, taxpayer-backed incentive plan for the company to expand. The state required the company to create 2,000 new jobs — 700 by 2007 and another 1,300 by 2012.