Brandon Bell/Getty Images On May 25, 2020, the world witnessed the final moments leading up to George Floyd’s death: Police officer Derek Chauvin pinned a handcuffed Floyd to the ground with his knee for more than nine minutes until he became unresponsive. The bystander video of the incident, with Floyd muttering his last words, “I can’t breathe,” sparked racial justice protests around the globe. Now, nearly a year later, Chauvin is on trial facing charges of second-degree unintentional murder, third-degree murder, and second-degree manslaughter. But despite what people saw, both virtually and in person last May, at the center of the trial is the question: What ultimately killed Floyd? The prosecution has argued that it was Chauvin’s knee, constricting Floyd’s neck and airway, that ultimately led to his death. Meanwhile, the defense has argued that it was Floyd’s history of drug use and underlying conditions that caused his death.