When lawyers were preparing to defend against a lawsuit over a death in police custody in Fresno, Calif., they knew whom to call. Dr. Gary Vilke has established himself as a leading expert witness by repeatedly asserting that police techniques such as facedown restraints, stun gun shocks and some neck holds did not kill people. Officers in Fresno had handcuffed 41-year-old Joseph Perez and, holding him facedown on the ground, put a spinal board from an ambulance on his back as he cried out for help. The county medical examiner ruled his death, in May 2017, a homicide by asphyxiation. Dr. Vilke, who was hired by the ambulance provider, charged $500 an hour and provided a different determination. He wrote in a report ... that Mr. Perez had died from methamphetamine use, heart disease and the exertion of his struggle against the restraints. Dr. Vilke ... is an integral part of a small but influential cadre of scientists, lawyers, physicians and other police experts whose research and testimony is almost always used to absolve officers of blame for deaths, according to a review of hundreds of research papers and more than 25,000 pages of court documents, as well as interviews with nearly three dozen people. Their views infuriate many prosecutors, plaintiff lawyers, medical experts and relatives of the dead, who accuse them of slanting science, ignoring inconvenient facts and dangerously emboldening police officers to act aggressively. Many of the experts also have ties to Axon, maker of the Taser.