Federal immigration agents said they uncovered 350 suspected undocumented workers in a raid on a Mississippi electrical equipment plant Monday, hours after sealing all entrances amid reports their sweep had idled normal operations.

Barbara Gonzalez, a U.S. Immigration and Customs Enforcement spokeswoman, confirmed the raid and said it targeted Howard Industries Inc. of Laurel.

The company produces dozens of products ranging from electrical transformers to medical supplies, according to the company's Web site.

"This is a targeted enforcement operation that is part of an ongoing ICE investigation that has revealed that illegal aliens are employed at Howard Industries," Gonzalez said, adding late Monday that agents were still interviewing plant workers.

She declined to say how many federal agents were involved, but said they acted on a tip provided by a union worker.

Another agency spokesman, Brandon Montgomery, told The Associated Press outside the plant Monday afternoon that agents were talking with everyone who worked at the sprawling plant to determine their residency status.

All plant entrances were blocked, with tents set up at some ICE checkpoints to keep agents out of a steady rain. Motorists traveling on roads behind the plant were stopped by officers in unmarked vehicles and told to leave.

Suspected illegal workers were loaded later Monday into white vans with shaded windows and driven away as ICE agents guarded the plant entrances. Gonzalez wouldn't say where they were headed other than to say they were being taken to a holding facility.

People leaving the plant told The Hattiesburg American newspaper that so many workers were stopped that operations were shut down. It wasn't clear how many workers the plant employed.

A recording at Howard Industries plant on Monday said the telephone switchboard was closed.

Billy Howard, the company's chief executive officer, did not immediately respond to a message left by The AP. A man who answered a phone call at the company's security station said reporters would have to call back Tuesday.

Howard Industries was founded in the 1960s. In 2002, state lawmakers approved a $31.5 million, taxpayer-backed incentive plan aimed at helping to expand its operations.

The raid is one of several nationwide in recent years.

On May 12, federal immigration officials swept into Agriprocessors, the nation's largest kosher meatpacking plant, in Iowa. Nearly 400 workers were detained and dozens of fraudulent permanent resident alien cards were seized from the plant's human resources department, court records showed.