GREENVILLE, S.C. – Federal agents detained more than 300 suspected illegal immigrants Tuesday in a raid at a chicken processing plant that has been under investigation for months.

The raid took place during a shift change, when police and federal agents spread through the House of Raeford's Columbia Farms plant and ordered all workers to show identification, according to officials and witnesses.

Maria Juan, 22, was one of about 50 relatives and friends of workers who huddled at the edge of the plant after the raid, some weeping and others talking frantically on cell phones. She was seeking information about her 68-year-old grandmother, a legal immigrant from Guatemala who went to work without identification papers but was later released.

“Families are going to be broken apart,” Juan said. “There will be kids and babies left behind. Why are they doing this? Why? They didn't do anything. They only wanted to work.”

Immigration officials kept the workers inside the plant and spent most of the morning trying to interview them and figure out how many are in the U.S. illegally, Assistant U.S. Attorney Kevin McDonald said.

The number could be large. A recent review found that immigration paperwork for more than 775 of 825 workers contained false information, McDonald said. Immigration agents scoured the plant for paperwork and other information for the investigation.

House of Raeford processes chickens and turkeys in eight plants in North Carolina, South Carolina, Georgia, Louisiana and Michigan. A sales manager at the Greenville plant referred questions to the company's Rose Hill, N.C., headquarters, where a woman answering the phone said there was no immediate comment.

Federal prosecutors and immigration agents have been investigating the plant's hiring practices for several months. Twelve people have been charged, most accused of falsifying documents. Seven have pleaded guilty, three are awaiting trial and two have fled, McDonald said.

The Charlotte Observer first reported in February that plant workers were in the country illegally and company managers knew it.

One plant worker backed up that account Tuesday.

“Everyone knew most of the workers were illegal. It was no secret. We just came in and did our work and you kept to yourself,” said Dorothy Anthony, who works with sister Alice on the deboning line.

The women, both American citizens, were released after showing ID.

Many workers live near the plant and drivers in the neighborhood stopped Tuesday to ask about them.

Officials are arranging to care for the children of workers detained in the raid, one of several nationwide this year.

In August, more than 600 suspected illegal immigrants were detained at a Mississippi transformer plant in the largest single-workplace immigration raid in U.S. history. And in May, 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, according to court records.