(CNN) -- Federal immigration agents arrested about 300 workers Tuesday in a raid at a poultry processing plant in Greenville, South Carolina, the Department of Justice said.
The agents executed a criminal search warrant at 9 a.m. at the Columbia Farms poultry processing plant, capping a 10-month investigation into the plant's employment practices, said Barbara Gonzalez, a spokeswoman for Immigration and Customs Enforcement.
About 58 of those arrested were allowed to return to their homes to take care of their children or for other humanitarian reasons, she said. The others were to be held in an ICE detention facility in the area.
"They are all illegals," Gonzalez said. "We have charged them with being in violation of U.S. immigration laws."
The investigation has already resulted in criminal charges being filed against 11 supervisors and a human resources manager, she said.
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, The Associated Press reported.
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, the AP reported.
"Families are going to be broken apart," Juan told the AP. "There will be kids and babies left behind. Why are they doing this? Why? They didn't do anything. They only wanted to work."
No one from Columbia Farms or from its parent company, House of Raeford Farms in Raeford, North Carolina, responded immediately to telephone messages.
The Charlotte Observer newspaper first reported in February that plant workers were in the country illegally and company managers knew it, the AP reported.
The raid "is a drop in the bucket" that is unlikely to persuade anyone in the United States illegally to go home, said Dan Kowalski, an Austin, Texas-based lawyer specializing in immigration law.
He questioned the conclusion by Gonzalez and ICE that all of those arrested are indeed illegal immigrants.
"A judge has to say that, they can't just say that," he said.