A nationally renowned drug rehab program in Texas and Louisiana has sent patients struggling with addiction to work for free for some of the biggest companies in America, likely in violation of federal labor law.

The Cenikor Foundation has dispatched tens of thousands of patients to work without pay at more than 300 for-profit companies over the years. In the name of rehabilitation, patients have moved boxes in a sweltering warehouse for Walmart, built an oil platform for Shell and worked at an Exxon refinery along the Mississippi River.