We are all witnesses. By now, we have all had the opportunity to see the video in which Minneapolis Police Officer Derek Chauvin killed George Floyd by kneeling on his neck for nearly nine minutes.

Most of us have heard the recording of Floyd’s voice gasping, “Please, I can’t breathe.” We have seen Chauvin’s ice-cold lack of concern as he continued to keep his weight bearing down on the man lying face down and handcuffed, knowing that the whole incident was being captured on camera by an onlooker with a cellphone. If Chauvin – who has been fired and charged with murder – were worried about what our reaction would be, he didn’t show it.

And since then we have seen the outpouring of grief and rage that has filled the streets in cities coast to coast and overseas in protest. Whether we were among the millions of mostly nonviolent protesters who chanted that “Black Lives Matter,” or whether we watched it from a safe distance, we can’t say we didn’t know it was happening. We know that these marches are not a response to a single isolated incident, but a well-established pattern of officially sanctioned violence against African Americans that goes unpunished year after year. We are witnesses to this, too.

Seeing what we’ve seen, our response has consequences, even if we do nothing. At least three Minneapolis police officers saw what Chauvin was doing and did not stop him. They may end up in prison for their failure to act. Most Americans didn’t see what happened to Floyd until it was too late to stop it. But what about the next killing, or the one after that?

In 1965, Martin Luther King Jr delivered a eulogy for James Reeb, a white Unitarian minister from Boston who died days after a vicious beating he received for joining the civil rights march in Selma, Alabama.

It was natural to wonder “who” killed Reeb, King said that day (the case is still officially unsolved). But the more important question is “what” killed him.

Reeb’s death was caused by politicians who feed their “constituents the stale bread of hatred and the spoiled meat of racism,” King said. “He was murdered by the brutality of every sheriff and law enforcement agent who practices lawlessness in the name of the law.”

But King also pointed the finger at the indifference of ministers who did not speak out against injustice; the timidity of the federal government, which would send troops to Vietnam but would send none to protect its citizens at home, and the cowardice of everyone who stood on the sidelines and accepted an evil system.

“So in his death, James Reeb says something to each of us, black and white alike – says that we must substitute courage for caution, says to us that we must be concerned not merely about who murdered him but about the system, the way of life, the philosophy which produced the murder.”

What will it take to change that system? We don’t have the answers. But as Americans who have witnessed this national agony for too long, we have an obligation to find them.