DENVER – U.S. prosecutors have charged brothers Eric and Ryan Jensen, former owners of Colorado-based Jensen Farms, with six counts each of introducing adulterated food into interstate commerce, tied to shipping tainted melons to markets in 2011.

The farmers’ listeria-contaminated cantaloupes killed 33 people in 2011, after they began washing the farm’s cantaloupes with devices used to clean potatoes and failed to use a chlorine spray feature that kills deadly bacteria.

Both men pleaded guilty in U.S. District Court in Denver on Tuesday, Oct. 22, to six counts of adulteration of a food and aiding and abetting. Each faces not more than one year in federal prison and a fine of up to $250,000 per count.

Forrest Lewis, attorney for Eric Jensen, said the brothers thought the cleaning operation they used “was safe and inspected and adequate.”

In addition to the deaths, the listeria outbreak linked to the farm in the southeastern corner of Colorado led 147 people across 28 states to be hospitalized, authorities said. One woman suffered a miscarriage.