A yearlong effort to hold someone criminally responsible for the death of Freddie Gray came to an abrupt end on Wednesday, with prosecutors in Baltimore announcing they would be dropping all charges against three police officers who were scheduled to be tried this summer and fall.

Gray died in police custody in April 2015 after being shackled in a police transport van without a seatbelt and incurring a severe spinal cord injury while the van was driven to central booking. Baltimore State’s Attorney Marilyn J. Mosby won praise from supporters of the Black Lives Matter movement in May 2015 when she moved with unusual swiftness to bring charges against six police officers who interacted with Gray before and after he sustained his injury.

The state’s argument was that the officers were responsible for Gray’s death because they failed to secure him in the police van and failed to get him medical help when it became clear that he was injured. Though the details differed from case to case, the broad outline of the prosecution’s legal theory was that decisions the officers made led to Gray’s death, these decisions were intentional and unreasonable, and therefore criminally negligent and reckless.

Three of those officers have been acquitted in bench trials by Judge Barry Williams in recent months; a hung jury resulted in a mistrial for a fourth. Wednesday’s announcement that the remaining charges will be dropped came at the start of what would have been the fifth trial in the Gray case, this one centered on whether Officer Garrett Miller broke any laws while carrying out Gray’s detention and arrest.

In a press conference held Wednesday morning across the street from where Gray was arrested, a defiant Mosby said she stands behind her decision to charge the officers and remains convinced of the legal arguments that members of her team presented in court. She also alleged that members of the Baltimore Police Department had demonstrated an “obvious bias” toward their colleagues during the investigation of Gray’s death and had been “completely uncooperative” with her office’s efforts. While it’s not surprising that Mosby didn’t have many fans in the Baltimore Police Department, her accusations Wednesday were nevertheless striking: As Baltimore Sun reporter Kevin Rector noted on Twitter, “She is alleging massive collusion.”

It has been clear for some time now—at least since the acquittal of police van driver Caesar Goodson, against whom the state arguably had the strongest case—that Mosby may have acted prematurely when, at an impassioned press conference following a period of destabilizing unrest in Baltimore, she declared her intention to prosecute the six officers. The charges initially struck many in Baltimore and around the country as a first step toward justice—an all-too-rare example of a prosecutor holding police officers accountable for deadly misconduct. As the Baltimore Sun wrote at the time, “After weeks of tension and occasional violence, it felt as if many exhaled in relief.” But it now appears that the evidence simply wasn’t there to convict any of the officers.

After four unsuccessful trials, many legal experts tracking the proceedings have come to believe that the state’s case against the officers was irredeemably thin—and that the criminal negligence argument was not working on Williams. As the New York Times noted earlier this month, Williams expressed skepticism during the trial of Lt. Brian Rice that Rice’s decision not to place restraints on Gray when he was in the police van should be seen as a criminal act. “You’re saying he could have done it, so the simple fact that he didn’t do it means he’s guilty of the crimes?” Williams asked.

As David Jaros, a University of Baltimore law professor, told the Sun, “The judge made it clear that just because you made a mistake—someone may have done something that was very wrong, that was very inappropriate—but that doesn’t necessarily make it a crime.” He added: “If you do something that is grossly unreasonable, but you’re not aware of the risk you’re taking, you’re not being grossly criminally negligent, even though you’re being negligent.”

Perhaps the biggest hurdle for prosecutors in these cases was that they had to prove their defendants acted illegally by failing to do something good—buckle Gray in, seek medical help—as opposed to actively doing something criminal. As University of Pittsburgh law professor David Harris told the Sun, “The American legal system is very reluctant to hold anybody, not just these police officers but anybody, criminally responsible for not doing something.”

The outcome of the criminal cases in the Freddie Gray case will undoubtedly be seen in some corners as a defeat for the cause of police accountability, as well as an insult to the idea that police officers should be held responsible when they cause someone’s death. But while it’s true that Gray died as a result of an injury sustained in police custody, this was never a straightforward case—in part because investigators couldn’t even conclusively determine which of the officers caused Gray’s injury, when, or how.

In no way does the lack of criminal convictions in the Freddie Gray case mean we should excuse the bad decisions made by the police officers who were responsible for his well-being. But in evaluating whether Mosby’s failure represents a step back in the movement to make America’s police officers more conscientious and less violent, it’s worth taking stock of just how difficult, even by the standards of police prosecutions, this case was.

There is something undeniably intuitive about judging progress in police reform by counting the number of officers who have gone to prison for their misdeeds. This is why some of the largest protests we’ve seen since the dawn of the Black Lives Matter era have been in reaction to nonindictments, and it’s why the first question many activists and reformers now ask when officers kill unarmed people in the line of duty is, “Will they be prosecuted or not?” I myself have pointed to the huge spike in police prosecutions observed last year—at least 17 officers were indicted for murder or manslaughter in connection with on-duty shootings, up from an average of five per year over the previous decade—as evidence that the tide is turning toward greater accountability.

But the Freddie Gray trials—the ones that ended in acquittal as well as the ones that never happened at all—should serve as a reminder that the ultimate goal of the activism we’ve seen lately around police misconduct is not just to put cops behind bars but to reduce the number of people who die as a result of interactions with them. As Mosby’s experience with this case makes clear, proving criminal charges against officers who make fatally bad decisions on the job is a difficult way to do that, even when you have a team of prosecutors who view the conduct in question as deplorable—hardly a given in most cases. There are other ways that a death like Gray’s can lead to change, though: The attention his death has received makes it more likely that the next time a police officer makes an arrest and places someone in the back of a police van, he will remember that the choices he makes while doing so could end up being scrutinized, criticized, and condemned.

Though ultimately unsuccessful, and perhaps doomed to failure from the start, the state’s efforts to mount a prosecution in the Freddie Gray case are proof that society’s standards for how officers should treat citizens have begun to change. The case against the six officers charged by Marilyn Mosby wasn’t provable. But it could still be consequential.