A new congressional investigation has concluded the deadliest recorded outbreak of food-borne illness in U.S. history was caused, in part, by the system intended to prevent it.

Last year, at least 30 deaths were caused by listeriosis linked to cantaloupe from Jensen Farms in Colorado. Before the outbreak, Jensen Farms received a clean bill of health from auditors the company had hired.

But those auditors knew about problems at Jensen Farms and rather than keep melons safe, helped put in place practices that led to the outbreak, according  to the congressional report.

"It appears that the auditors who inspected Jensen Farms did more than simply overlook egregious food safety practices: They specifically recommended these practices," members of the House Energy and Commerce Committee wrote in a letter presenting the findings to FDA Commissioner Margaret Hamburg.

While the FDA is responsible for produce safety, the agency's inspectors are unable to visit more than once every few years. In the interim, the produce safety net is an unregulated patchwork of third-party auditors.

Jensen Farms had hired the auditing firm Primus Labs, which in turn subcontracted inspections of the cantaloupe operation to another company: Bio Food Safety.

That company, the lawmakers told Hamburg, "recommended the production equipment and process that Jensen Farms put in place for the 2011 growing season. These recommendations appear to have increased food safety risks."

Auditors had given Jensen Farms weeks of advance notice before inspections. When those inspections uncovered problems, the report found, auditors still gave Jensen Farms glowing reviews and failed to notify the FDA.

Cantaloupes are just the latest in a string of nationwide disease outbreaks blamed on weak auditing. Third-party auditors had given a clean bill of health to the Peanut Corporation of America before the company was linked to a salmonella outbreak suspected of sicking 700 people and killing nine. And in 2010, independent auditors had given Wright County Egg a glowing report just before the company was linked to nearly 2,000 illnesses.

Part of the problem, the lawmakers said in their letter to Hamburg, is that the for-profit system may result in some auditors worrying more about making money than protecting consumers.

"A failing audit has significant economic implications for the producer," they wrote. "To the extent an auditor applies more demanding food safety standards, it may be less likely to be hired."

The FDA does not currently regulate auditors in the United States, although it does have jurisdiction over those that inspect imports. Without reform of the rules, the investigators write, food safety crises are likely to recur.

"These problems are unlikely to be limited to Jensen Farms, however," they wrote. "The officials the committee interviewed indicated that the practices used at Jensen Farms are similar to those used in thousands of other food safety inspections."