A white police officer and a black teen met on a street. Minutes later, one was dead.

America sifted through dozens of witness accounts for months. The truth of what happened was lost in memory and obscured by bias. Nobody caught the fatal encounter between Officer Darren Wilson and Michael Brown on camera. The promise of perfect recall would forever prove elusive.

"Many questions remain about what happened leading up to the moment when our son was shot," Brown's parents wrote afterwards. But if Wilson had been wearing a camera, they said, "there would be no questions." They called for cameras on every officer in the country. President Barack Obama committed the federal government to the task.

There is significant evidence that cop cams cut down on most civilian complaints. But a close examination of violent encounters with the police caught on tape suggests that even with seemingly incontrovertible video evidence, questions will often linger. The kind of sea change that police reform activists desire will still likely escape them.

"I don't think anything is as good as having a camera," said John Burris, an Oakland civil rights attorney who represented both Rodney King and the family of Oscar Grant, who was killed by an officer in an Oakland train station while he was lying face-down on the ground. But, Burris added, "The realities of the world are police get the benefit of the doubt."