The owners of a Colorado cantaloupe farm were arrested Thursday on charges stemming from a 2011 listeria epidemic that killed 33 people in one of the nation’s deadliest outbreaks of foodborne illness.

They pleaded not guilty in federal court and were released on unsecured bonds. Trial is scheduled for Dec. 2.

Prosecutors said the federal Food and Drug Administration and the Centers for Disease Control and Prevention determined the Jensens didn’t adequately clean the cantaloupe.

Criminal charges in food poisoning cases are rare, said attorney William Marler, who represents many of the listeria victims in civil cases against Jensen Farms. Only four other people have faced such charges in the past decade, he said.

The FDA has said the melons likely were contaminated in Jensen Farms’ packing house. It concluded that dirty water on a floor, and old, hard-to-clean equipment probably were to blame.

The epidemic was the deadliest outbreak of foodborne illness in 25 years, and it delivered a serious blow to Colorado cantaloupe farmers.

A number of lawsuits were filed by people who were sickened or who had a family member die after the outbreak.

Eric Jensen, 37, and Ryan Jensen, 33, could face up to six years in prison and up to $1.5 million in fines each if convicted of all counts against them, prosecutors said.

The Jensens’ farm in southeastern Colorado filed for bankruptcy after the outbreak.

After Thursday’s hearing, the men released a statement calling the outbreak a "terrible accident" and saying they were shocked and saddened by it.