CNN exclusive: A meat processing plant in California was buying diseased dairy cows and processing them when government inspectors weren't there, federal officials tell CNN.

After the cows were killed, employees at Rancho Feeding Corp., based in Petaluma, California, 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, the officials say.

A tip from an employee led to a federal investigation and the recall of nearly 9 million pounds of meat processed by Rancho.

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