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.

The goods these prisoners produce wind up in the supply chains of a dizzying array of products found in most American kitchens, from Frosted Flakes cereal and Ball Park hot dogs to Gold Medal flour, Coca-Cola and Riceland rice. They are on the shelves of virtually every supermarket in the country, including Kroger, Target, Aldi and Whole Foods. And some goods are exported, including to countries that have had products blocked from entering the U.S. for using forced or prison labor.

Many of the companies buying directly from prisons are violating their own policies against the use of such labor. But it’s completely legal, dating back largely to the need for labor to help rebuild the South’s shattered economy after the Civil War. Enshrined in the Constitution by the 13th Amendment, slavery and involuntary servitude are banned – except as punishment for a crime.

That clause is currently being challenged on the federal level, and efforts to remove similar language from state constitutions are expected to reach the ballot in about a dozen states this year.

Some prisoners work on the same plantation soil where slaves harvested cotton, tobacco and sugarcane more than 150 years ago, with some present-day images looking eerily similar to the past. In Louisiana, which has one of the country’s highest incarceration rates, men working on the “farm line” still stoop over crops stretching far into the distance. {read}