Drivers in Fort Worth were stopped at a roadblock and directed into a parking lot where they were asked to submit blood and saliva samples after their breaths were passively tested.

NBC 5 in Fort Worth investigated claims from drivers that they’d been stopped by uniformed officers and offered money to submit samples. They found that the stops were part of a government research study working to determine the number of chemically impaired drivers in 30 different cities across the U.S.

The National Highway Traffic Safety Administration is apparently paying a government contractor – the Pacific Institute for Research and Evaluation based in Calverton, Maryland - around $7.9 million to conduct the survey over three years. Officials and researchers claim the stops and submissions of samples are voluntary, but witnesses say they were not and an investigation of the fine print of literature handed to participants revealed that people who were stopped had their breaths passively tested before they could offer their permission.

On Tuesday, a spokesperson for the Fort Worth Police Department confirmed that off-duty officers assisted the NHTSA in stopping drivers for the study.