The murder of George Floyd by Minneapolis police officers was captured on video, not once but half a dozen times. As we try to understand why a police officer continued compressing a man's neck and spine for minutes after he'd lost consciousness, we have footage from security cameras at Cup Foods, where Floyd allegedly paid for cigarettes with a counterfeit $20 bill. As we wrestle with the sight of three officers standing by as their colleague killed Floyd, we have footage from the cell phones of witnesses who begged the officers to let Floyd off the ground. In the murder trial of Officer Derek Chauvin, who was patrolling despite 17 civilian complaints against him and previous involvement in two shootings of suspects, his defense may hinge on video from the body cameras he and other officers were wearing.

None of these videos saved George Floyd's life, and it is possible that none of them will convict his murderer.

Officer Chauvin knew this. In the video shot by 17-year-old Darnella Frazier, you can see him lock eyes with the teenager. He knows she's filming, and knows that the video is likely being streamed to Facebook, to the horror of those watching it. After all, in a suburb of nearby St. Paul four years earlier, Officer Jeronimo Yanez shot and killed Philando Castile while Castile's partner streamed the video to Facebook. Yanez's police car dashcam also recorded the seven shots he pumped into Castile's body. He was charged and acquitted.

After Castile's death, I wrote a piece for MIT Technology Review about "sousveillance," the idea posited by the inventor Steve Mann, the "father of wearable computing," that connected cameras controlled by citizens could be used to hold power accountable. Even though bystander video of Eric Garner being choked to death by New York police officer Daniel Pantaleo in 2014 had led not to Pantaleo's indictment but to the arrest of Ramsey Orta, the man who filmed the murder, I offered my hope that "the ubiquity of cell-phone cameras combined with video streaming services like Periscope, YouTube, and Facebook Live has set the stage for citizens to hold the police responsible for excessive use of force."

I was wrong.