Word of the recall came Sunday, weeks after authorities asked suppliers, schools and processors to set the meat aside. Hundreds of schools across the state are thought to have received ground beef from the Westland Hallmark plant.
The recall dates back to meat packed in February 2006.
An undercover investigation by the Humane Society last fall found the meatpacker had been improperly using sick or injured animals that couldn't walk.
In an emergency conference call with reporters over the weekend, USDA undersecretary Richard Raymond said his agency didn't know how much of the meat was left.
"A lot of this is fresh, raw product and with ground beef, etc, has a very short shelf life and very short refrigerator life. So a great majority of it has probably been consumed," said Raymond.
Cattle that can't walk are considered at higher risk to pass on a variety of food-borne diseases, but no illnesses have been linked to the recalled beef so far.
It is the largest meat recall in U.S. history.