Police in the U.S. have been warned for decades that the common tactic of handcuffing someone facedown could turn deadly if officers pin them on the ground with too much pressure or for too long. What some officers are doing today conflicts with what has long been recognized as safe, a deadly disconnect that highlights ongoing failures in police training, an Associated Press investigation has found.

FRONTLINE and The Associated Press investigate deaths that occurred after police used tactics like prone restraint and other “less-lethal force.”