A hidden path to America’s dinner tables begins here, at an unlikely source – a former Southern slave plantation that is now the country’s largest maximum-security prison.

Unmarked trucks packed with prison-raised cattle roll out of the Louisiana State Penitentiary, where men are sentenced to hard labor and forced to work, for pennies an hour or sometimes nothing at all. After rumbling down a country road to an auction house, the cows are bought by a local rancher and then followed by The Associated Press another 600 miles to a Texas slaughterhouse that feeds into the supply chains of giants like McDonald’s, Walmart and Cargill.

Intricate, invisible webs, just like this one, link some of the world’s largest food companies and most popular brands to jobs performed by U.S. prisoners nationwide, according to a sweeping two-year AP investigation into prison labor that tied hundreds of millions of dollars’ worth of agricultural products to goods sold on the open market.

They are among America’s most vulnerable laborers. If they refuse to work, some can jeopardize their chances of parole or face punishment like being sent to solitary confinement. They also are often excluded from protections guaranteed to almost all other full-time workers, even when they are seriously injured or killed on the job.