On October 14, 2007, a middle-aged man was growing irate at the arrivals terminal of the Vancouver International Airport. Robert Dziekanski had been stuck in the airport for 10 hours with a visa problem, unable to find his older mother. He couldn’t speak English—couldn’t speak any language other than Polish. So, frustrated or delirious, he had begun throwing furniture to the ground.

Four members of the Royal Canadian Mounted Police arrived to investigate the commotion. One of them gave Dziekanski an instruction in English. It’s unclear whether he complied, and it hardly mattered: Less than 25 seconds after getting to the scene, Corporal Benjamin Robinson, the senior officer present, ordered his team to use a stun gun on Dziekanski.

The police tased Dziekanski five times. He screamed, collapsed, and writhed on the floor. Instead of letting him sit up to catch his new breath, the officers forced his face against the ground as they tried to handcuff him. He resisted, then stopped fighting. When paramedics arrived minutes later, they could not revive him. Dziekanski’s heart had stopped.

A couple hundred feet away, a Canadian named Paul Pritchard was recording the incident on his phone. His video—which was confiscated by police, and which he only reclaimed through litigation—did not match the police report.

The police and Pritchard’s video were so different as to resemble alternate worlds. The police had claimed three men approached Dziekanski. The video showed four. The police had claimed that Dziekanski was reaching for something on a desk when he was tased, as if preparing to assault the officers. The video showed he wasn’t. The police said they couldn’t have pepper-sprayed Dziekanski, as too many other people were nearby. The video showed that a plexiglass wall separated Dziekanski and the officers from anyone else in the airport.

The video made a difference.

Just last month, Benjamin Robinson was found guilty of perjury.

* * *

Three days ago, a local newspaper in South Carolina reported the police explanation for the shooting death of an unarmed black man, Walter Scott, as he fled from a routine traffic stop in the town of North Charleston. From The Post and Courier:

A statement released by North Charleston police spokesman Spencer Pryor said a man ran on foot from the traffic stop and an officer deployed his department-issued Taser in an attempt to stop him.

That did not work, police said, and an altercation ensued as the men struggled over the device. Police allege that during the struggle the man gained control of the Taser and attempted to use it against the officer.

The officer then resorted to his service weapon and shot him, police alleged.

The Post and Courier also quoted a confused cousin of Scott’s: “He’s not a violent guy—never seen him argue with anybody. I just can’t see it,” Samuel Scott told the paper.

Then, on Tuesday, both The Post and Courier and The New York Times obtained videos of the shooting. They showed a different story.

Screenshot from a video obtained by The New York Times

The video shows the officer, Michael T. Slager, shooting Scott as he runs away. Slager does not start firing—eight times, at Scott's back—until Scott is fleeing. The video then shows Slager dropping an object on Scott’s body. It isn't clear whether this object is a stun gun. Finally, the video shows sets of officers arriving on the scene in the minutes after the shooting, and it shows none of them attempting CPR.

On Tuesday evening, Slager was charged with murder.

* * *

In the past year, after the killings of Michael Brown and Tamir Rice, many police departments and police reformists have agreed on the necessity of police-worn body cameras. Body cams seem to be good for everyone. By creating a better record of officer-citizen interactions, the argument goes, they protect cops from unfounded accusations and give citizens a kind of justice insurance policy.

But the most powerful cameras aren’t those on officer’s bodies. They’re those wielded by bystanders.

We don’t yet know who shot these videos of Slager, who carried these tools of witness. But unknown cameramen and women lived out high democratic ideals: They watched a cop kill someone, shoot recklessly at someone running away, and they kept the camera trained on the cop. They were there, on an ordinary, hazy Saturday morning, and they chose to be courageous. They bore witness, at unknown risk to themselves.

Which isn’t to say that civilian-held cameras are always effective at securing a conviction. Despite the fact that the world can now see Eric Garner being killed by an illegal chokehold—despite the fact that New York City Police Department banned chokeholds years ago—film of the incident did not result in the officer, Daniel Pantaleo, being charged. But thanks to the efforts of Ramsey Orta, who filmed Garner’s death, we know.

“We have been talking about police brutality for years. And now, because of videos, we are seeing just how systemic and widespread it is,” tweeted Deray McKesson, an activist in Ferguson, after the videos emerged Tuesday night. “The videos over the past seven months have empowered us to ask deeper questions, to push more forcefully in confronting the system.”

“Pics or it didn’t happen” is a terrible motto for a justice system. Nor can accountability for police officers rely on someone always standing nearby with a smartphone. But the process of ascertaining the truth of the world has to start somewhere. A video is one more assertion made about what is real.  Today, through some unknown hero’s stubborn internal choice to witness instead of flee, to press record and to watch something terrible unfold, we have one more such assertion of reality.