Last winter, Dr. Gail Van Norman sat on the witness stand in the federal courthouse in Oklahoma City, testifying as part of a trial that would determine whether Oklahoma’s lethal injection procedure was constitutional. Two weeks earlier, at the request of law…

In the execution chamber, she testified, Postelle was lying face-up on a gurney with his arms stretched out beside him. Executioners injected him with midazolam, a drug that was supposed to knock him…