The common expression is that someone “committed a crime.” Ever read that someone “omitted a crime”? That’s what three police officers in the sideshow of the most notorious police killing of the decade, George Floyd, now stand convicted of, the crime of omission.

A federal jury determined that the officers — Tou Thao, 36; J. Alexander Kueng, 28; and Thomas Lane, 38 — had willfully violated Mr. Floyd’s constitutional rights by not providing medical care when he lost a pulse and that two of them were also guilty of not intervening to stop a fellow officer, Derek Chauvin, from planting his knee on Mr. Floyd’s neck.

Mr. Kueng and Mr. Lane both helped Mr. Chauvin restrain Mr. Floyd while he was handcuffed facedown on the pavement. Mr. Thao stood nearby, keeping bystanders away.

The “willful” violation of Floyd’s constitutional right to live was predicated not by what they did, but what they didn’t do, by not providing medical care, by not stopping Chauvin from killing Floyd. Their crime was failing to intervene.

Legal experts and racial justice activists had been closely watching the case, saying it could have more ramifications for policing than even Mr. Chauvin’s murder convictions. At the heart of this case was a more widespread problem, experts say, than a single officer’s act of violence: the tendency of officers to stand by when they witness a fellow officer committing a crime.

This has, indeed, been a core problem with police culture, and one that proves the lie of the “one bad apple” argument. Sure, one cop did the affirmatively bad act, but it could not have happened or been covered up without the willing acquiescence of other officers who knew, saw and did nothing. Whether they are as “guilty” as the perp cop may be subject of debate, but they are certainly not innocent of wrongdoing when they were complicit in the offence by their inaction and aid in concealing the wrong.

But applying this to the myriad wrongs committed by police officers isn’t necessarily a clear duty. Beating a cuffed perp is an easy call, as is planting evidence or a weapon, lying on a police report or a blatant execution. As with most things law, however, it can get far more squishy at the edges, and under fluid circumstances where the line isn’t obvious, any more than it’s obvious at the moment the line gets crossed. And the summation by prosecutors in this case reflected this, when they used the time-tested appeal to “common sense” rather than evidence.

But prosecutors tried to portray the actions of the onlookers as a powerful counterpoint to what the officers were doing — arguing that if a small crowd of average citizens could see clearly that Mr. Floyd was facing a dire medical emergency, then three well-trained police officers surely should have.

“You just need plain old common sense,” Manda Sertich, a prosecutor, told the jury in her closing argument. “And you just need plain old human decency.”

There was a line to be drawn in this prosecution, where George Floyd went from conscious person to unconscious, when he no longer had a pulse. What blurred that line was the paramilitary hierarchy and reliance on the experience of the one cop with time on the job, Derek Chauvin.

The verdict was also a rejection of the argument cited in each of the three defense cases: that the officers trusted Mr. Chauvin, the senior officer on the scene, and therefore were not aware that what he was doing was illegal.

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As Mr. Floyd was being killed, Mr. Thao, a veteran officer who was Mr. Chauvin’s partner, was keeping a crowd of bystanders at bay. Mr. Lane and Mr. Kueng, both rookies, were helping Mr. Chauvin detain Mr. Floyd after a convenience store clerk said he had used a fake $20 bill to buy cigarettes.

Thao was Chauvin’s partner, but his attention was turned toward the increasingly angry crowd, the same one of “average citizens” who the prosecution argued could see that Floyd was “facing a dire medical emergency,” an argument of some emotional appeal but dubious merit. As for the other two rookies, at what point should it have been sufficiently clear that their senior officer was not only wrong in his judgment call that Floyd was fine, but went from restraining Floyd to killing him? At what point do they act, and what actions do they take? Get off Floyd, call an ambulance, begin efforts to revive Floyd, forcibly remove Chauvin if their senior officer refuses to get off Floyd’s neck on his own?

During the trial, the defense lawyers tried to absolve the three officers, with Robert Paule, who represents Mr. Thao, telling the jury during his closing statement that “Just because something has a tragic ending does not mean it’s a crime.”

Had George Floyd not died that day, but had either survived on his own or been revived by EMTs, it is highly unlikely that anyone would know Floyd’s name and likely that these three cops would still be on the job, carrying a gun and shield. But the conduct in which they engaged, or failed to engage, would have been no different. Tragic endings do not mean a crime has occurred. Lack of tragic endings does not mean a crime hasn’t occurred.

If the conduct, whether by commission or omission, constitutes the crime, the outcome only matters as to degree. In this case, the outcome was not only tragic, but the conduct occurred at a particular moment in time when it became the focus of a nation. There have been far, far worse police abuses, far more clear acts of outrageous violence, that somehow didn’t manage to register at all on the public’s consciousness.

This one caught fire and so it became a cause célèbre for Black Lives Matter. That it ended in death left little doubt that these officers would held to account for the killing of George Floyd, for what they did or didn’t do. Whether this conviction reflects a change in cop culture to demand accountability for failure to act or just one outlier case is unclear, as is where the line is to be drawn going forward for cops to take action against other cops, senior cops, when the wrong isn’t flagrant.

At least here there was a line, when George Floyd lost consciousness, even if the line was more obvious after the fact than in the moment. And that line will be the subject of the next trial, the state prosecution for aiding and abetting Chauvin’s murder, because cases that capture the public’s attention are now two-trial cases, just so everyone realizes how much prosecutors care and love them back. After all, it’s only common sense to milk the prosecution when the public adores you for it.