The murder trial of former Minneapolis Police officer Derek Chauvin has begun, 10 months after George Floyd died in the street with Mr. Chauvin’s knee on his neck.

Americans, especially Black Americans, are watching closely to see if justice will be done.

The beginning of the trial made me profoundly sad, but also profoundly hopeful.

It made me sad because Mr. Floyd is gone and nothing will bring him back to his family.

It made me hopeful because Mr. Floyd’s killing sparked a renewed movement that is challenging police killings of unarmed civilians — a multiracial and multigenerational movement.

The fact that Mr. Chauvin has been indicted and is facing trial is itself reason for hope.

But my hope is tempered by history. As Black Americans know all too well, our system often fails to hold law enforcement accountable for killing Black people, even when the killing is on video.

Think about Eric Garner. Mr. Garner died in 2014 after a New York City Police officer put him in a choke hold and wrestled him to the ground. Before he died, Mr. Garner told police officers, “I can’t breathe,” 11 times. It was all on video.

But a New York grand jury decided not to indict the officer who killed Mr. Garner. And five years later, in 2019, U.S. Attorney General William Barr decided not to bring federal civil rights charges against him.

Last year, Mr. Floyd said “I can’t breathe” 27 times as he laid on the ground with Mr. Chauvin’s knee on his neck. Other officers looked on even as people begged and pleaded with them to spare Mr. Floyd’s life.

The trial testimony has been brutal. We learned on the first day that Mr. Chauvin kneeled on Mr. Floyd’s neck for nine and a half minutes — even longer than we thought. On the second day, a teenage witness tried to hold back tears on the stand while she described watching a man die and not being able to help him.

I am convinced that the truth about what happened to George Floyd came out only because someone filmed his death. It was only after video was shared widely that the officers were fired and charges eventually were filed. And even now, rumors and disinformation being spread online blame Mr. Floyd for his own death.

A New York Times investigation published last year found that during the past decade, at least 70 people — more than half of them Black — died in law enforcement custody after saying “I can’t breathe.” Most of them were stopped or held for nonviolent infractions, 911calls about “suspicious” behavior or mental health issues. More than 40 of the “I can’t breathe” deaths happened after Mr. Garner was killed.

We hope and expect that the trial now under way will bring justice for Mr. Floyd and his family. But obviously the problem in law enforcement is much bigger than individual “bad apples.”

We can save lives and prevent future deaths by reimagining public safety and demanding real, institutional change. That is the motivation for measures like the federal George Floyd Justice in Policing Act and a wave of initiatives being proposed and considered at state and local levels.

We must finally have equal justice under the law, and it starts with a criminal justice system that holds law enforcement accountable.