(CNN) PETALUMA, CALIFORNIA – Earlier this year, a dusty little slaughterhouse in Northern California was ground zero for one of the biggest meat recalls in years. Rancho Feeding Corp. had called back nearly 9 million pounds of bad meat from thousands of unsuspecting stores across the country.

The story of how millions of pounds of bad meat – products the U.S. Department of Agriculture called “unfit for human food” – made it out into the world and triggered a criminal investigation is one of staggering deception and cancerous cows, federal officials familiar with the investigation tell CNN. And the plant where it all went down was also the setting for an illicit romance, according to documents obtained by CNN.

Federal investigators started surveillance on the California facility after getting a tip from a former Rancho employee. In January, federal marshals raided the Petaluma plant and seized the company's records. Days later, the first recall notice went out, officials said.

Investigators now believe that Rancho was buying diseased dairy cows and processing them when government inspectors weren’t there. After the cows were killed, employees would hide the warning signs of cancer by trimming off diseased parts, using a fake stamp of approval or even replacing the heads of sick cows with ones from healthy animals. It’s unclear which employees were involved, officials said.

The account provides a fuller picture of what happened inside the plant than has been previously made public. In a letter a day after the first recall, the USDA told Rancho an investigation found that the facility “shipped adulterated and misbranded product” and hadn’t inspected cattle that “were likely affected with epithelioma of the eye (eye cancer).”

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But that wasn’t the only misconduct going down at the plant. Turns out that one of the government inspectors – someone responsible for protecting consumers from bad meat – was having a romantic relationship with a plant foreman, according to a USDA email obtained by CNN.