The U.S. Justice Department has launched a criminal investigation into allegations that top poultry processors colluded to keep prices artificially high.

The probe came to light after federal attorneys sought to intervene in a long-running lawsuit in which customers accused chicken processors, including Tyson Foods Inc., Pilgrim’s Pride Corp., Sanderson Farms Inc. and Perdue Farms Inc., of illegally cooperating. The processors have denied the accusations.

The...