Phoenix - US immigration agents have arrested 595 people at a Mississippi factory in what was the largest workplace enforcement raid in the United States to date, an immigration official said on Tuesday.

    Immigration and Customs Enforcement spokeswoman Barbara Gonzalez said federal agents arrested the workers in a raid at the Howard Industries Inc. factory in Laurel, Miss, on Monday.

    "This is the largest targeted workplace enforcement operation we have carried out in the United States to date," Gonzalez told Reuters by telephone.

    The swoop at the plant, which makes electrical equipment including transformers, was part of an ongoing investigation into identity theft and fraudulent use of Social Security and for illegal immigrants.

    The nationalities of the detainees was not immediately available.

    Gonzalez said 475 detainees were transferred to an ICE detention facility in Jena, Louisiana, while nine unaccompanied minors - eight males and one female - were placed in the custody of the office of refugee resettlement.

    A further 106 people were released based on "humanitarian concerns," Gonzalez said.

    Immigration, particularly what do about some 12 million illegal immigrants living in the shadows in the United States, is a divisive issue in this election year.

    Last year, Republicans in the U.S. Senate killed an immigration overhaul that would have offered many workers in the U.S. illegally a path to citizenship.

    The measure was supported by both Republican presumptive nominee John McCain and Democrat rival Barack Obama.

    Since its failure, immigration authorities have stepped up workplace enforcement raids, which have nabbed more than 4,000 people nationally since October 1 last year.

    Previously the largest single immigration sweep was conducted at Agriprocessors Inc, a kosher meat plant in Postville, Iowa, which arrested 390 people in May.

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    Reporting by Tim Gaynor, editing by Cynthia Osterman.