Two Colorado brothers who grew listeria-contaminated cantaloupe in an outbreak that killed 33 people nationwide and sickened hundreds more were criminally charged Thursday by federal authorities.

Eric Jensen, 37, and Ryan Jensen, 33, turned themselves in to federal marshals and pleaded not guilty to six misdemeanor counts of introducing adulterated food into the food supply. The fourth-generation family farmers from Holly, in southeastern Colorado, were shackled at the wrists during their federal court hearing.

The outbreak two years ago this month was linked to Jensen Farms after an investigation traced half-eaten cantaloupe taken from patients’ refrigerators, to grocery stores and then to the farm. The Centers for Disease Control and Prevention officially has linked 33 deaths and a miscarriage to the outbreak, although 10 other people who had been infected with listeria bacteria after eating Jensen cantaloupe also have died.

The federal charges each carry up to one year in prison and a $250,000 fine. A trial date was set for Dec. 2.

The Jensens’ attorneys emphasized that the farm has cooperated with investigators all along; neither side would comment on whether a plea bargain is under negotiation.

“The charges against Eric and Ryan Jensen do not imply that they knew, or even should have known, that the cantaloupes had been contaminated,” said a statement issued by their lawyer. “As they were from the first day of this tragedy, the Jensens remain shocked, saddened, and in prayerful remembrance of the victims and their families.”

The brothers declined to comment.

Many widespread cases of foodborne illness go without prosecution of growers or distributors. U.S. Attorney spokesman Jeff Dorschner said the Jensen case stood out because of “the magnitude of the number of people who were hospitalized and who died, and it involved 28 of the 50 states.”

Victims and food-safety experts expressed surprise and satisfaction that charges had been brought from the 2011 outbreak. They said highlighting sloppy practices would send the system a message about food safety.

Jennifer Exley, whose father, Herb Stevens, was sickened by a contaminated cantaloupe and died in July, went to the hearing because she said she wanted to see the Jensen brothers in person. She has never spoken to them. The two farmers awaited their arraignment in a jury box with a handful of other men brought to court in chains.

“I wanted to humanize this for me,” Exley said outside the federal courthouse.

“I’m glad they were charged. I don’t think they did anything on purpose, but I think they had very sloppy farming practices.”

Victims, attorneys and food- safety experts have speculated since 2011 whether what happened at Jensen Farms would lead to any federal court action. Past prosecutions in food safety cases have proved mercurial and often took years from the time consumers were first sickened by pathogens.

“This is an unprecedented thing; it’s not the norm,” said Amanda Hitt, an attorney with the Government Accountability Project, a whistleblower-supporting group in Washington, D.C.

Civil lawsuits by victims are not enough to persuade food companies to clean up, Hitt said.

Food safety attorney Bill Marler of Seattle, who represents most of the victims of the listeria outbreak, praised the news but said prosecutors should consider going after the grocery stores and others.

“We will never have safe food from ‘farm to fork’ until the entire chain of distribution is held accountable for the food that they make a profit from,” Marler said.

No public information had surfaced that Ryan or Eric Jensen knew they had any safety problems. Experts who have watched past food safety outbreaks said federal prosecutors do not usually pursue a case unless there is clear evidence that owners knew of or ignored obvious signs of contamination.

“As this case so tragically reminds us, food processors play a critical role in ensuring that our food is safe,” U.S. Attorney John Walsh said in a statement. “They bear a special responsibility to ensure that the food they produce and sell is not dangerous to the public.”

Federal prosecutors can bring misdemeanor-level cases based on the simple fact of distributing contaminated food, regardless of who knew what or when they knew it. But they rarely do so, saving their firepower for wider felony cases with a “knowing” element of malice.

The Jensens, by contrast, told experts the changes they made between 2010 and 2011 were meant to make food safer. Federal guidelines at the time were not clear, but Thursday’s charges quote an FDA official saying the Jensens should have used a chlorine bath to kill bacteria.

FDA inspectors said their sorter was meant for raw potatoes and allowed dirty water to pool and recontaminate melons.

Numerous lawsuits have been brought by victims against the Jensens, their distributors, auditors and grocery stores. The Jensens eventually filed bankruptcy, and their insurance was put in a pool to compensate victims.

Michael Booth: 303-954-1686, mbooth@denverpost.com or twitter.com/mboothdp