IN THE SUMMER of 2013, Missouri criminal defense attorney Jennifer Bukowsky was preparing for an evidentiary hearing in the case of a pro bono client, Jessie McKim. The stakes were high: Along with his co-defendant, James Peavler, McKim had been convicted in 1999 of killing a woman named Wendy Wagnon and was serving life without parole at a maximum security prison. At the upcoming hearing, Bukowsky planned to argue that her client was innocent — and that the murder that sent him to die in prison was never a murder at all.

McKim was convicted in part based on the testimony of a local medical examiner, who claimed that the presence of petechiae on a dead body — small spots on the skin or the whites of the eyes where capillaries have hemorrhaged — is proof that a person was suffocated. But a toxicology report — completed after Wagnon's cause of death had already been determined as asphyxiation — revealed that Wagnon had lethal levels of methamphetamine in her system when she died.