BALTIMORE — The state’s attorney here dropped all remaining charges Wednesday against three city police officers awaiting trial in the death of Freddie Gray, closing the book on one of the most closely watched police prosecutions in the nation without a single conviction — and few answers about precisely how the young man died.

The announcement ended a sweeping, deeply polarizing prosecution that began last spring, as National Guard troops rumbled through the streets, with Baltimore under curfew and residents tense after looting and riots that broke out after Mr. Gray sustained a fatal spinal cord injury in police custody.

Mr. Gray, a 25-year-old black man, had been arrested after he spotted a police presence as he walked with friends and ran away. He was found unresponsive and not breathing after he rode unsecured in a police transport wagon after his arrest on a bright morning in April 2015, and died a week later. Six officers were charged with crimes including manslaughter and murder; the first trial ended in a hung jury, and three more officers were acquitted after trials before a judge.

Wednesday’s extraordinary turn put into sharp relief the wrenching national debate over race and policing, after a month of deadly shootings of black men and deadly retaliations against police officers around the nation. Just a few weeks ago, President Obama pleaded for racial healing after five police officers in Dallas were gunned down by a black Army veteran. The outcome also left the city deeply divided over whether its top prosecutor, Marilyn J. Mosby, 36, had overreached in her initial charges.

Facing cameras in front of a bright-colored mural — a homage to Mr. Gray — in the blighted West Baltimore neighborhood where he grew up, she defended herself, sounding every bit as fiery and passionate as she was a year ago in May when she drew national attention in announcing the charges. She accused the police department of working to thwart her investigation.

“We do not believe Freddie Gray killed himself,” Ms. Mosby said, calling the decision to drop the charges “agonizing.” Complaining she lacked an independent investigatory agency, she added, “Without real substantive reforms to the current criminal justice system, we could try this case 100 times and cases just like it, and we would still end up with the same result.”

But a police union official, Lt. Gene Ryan, and lawyers for the six officers struck back, with Lieutenant Ryan calling Ms. Mosby’s accusations “outrageous.” They argued that the judge, Barry G. Williams, who had prosecuted police misconduct while working for the Justice Department, had followed the evidence — even if Ms. Mosby did not like where it led.

“You can get a conviction against the police, whether a bench trial or a jury trial, if you do an investigation,” said one of the lawyers, Ivan Bates. But, he said, if “you quickly want to automatically say that the officers are guilty because they’re the police, then you perpetrate that fear that’s already there and that’s dividing our country.”

The exchanges showed that even in a majority-black city, with a black mayor and a black prosecutor, there are no easy answers to questions involving race and policing. The case featured a black victim and had a black judge. And three of the six officers are black, as is the defense lawyer who spoke on their behalf Wednesday.

In the end, there was anguish on all sides of the debate here and around the country.

“We’re nowhere,” Chuck Wexler, executive director of the Police Executive Research Forum, said in a telephone interview. “Both sides walk away from this feeling like they didn’t get justice — the people who were concerned about Freddie Gray, and the people who are concerned about cops doing their job.”

“We haven’t gotten to the bottom of the Freddie Gray case,” he said.

Despite the lack of convictions, Ms. Mosby, 36, argued that her work had not been for naught; there have been police department reforms, and the city is “one step closer to equality.” Officers now routinely buckle up prisoners traveling in police wagons, she said, and cameras record what happens inside.

In Baltimore, Wednesday’s news was met with grim resignation. Supporters and detractors of the police seemed, by this point, to expect the outcome. And it was clear that, more than a year after the arrest of Mr. Gray, whom officers said was carrying an illegal knife, deep divisions remained.

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In the Sandtown-Winchester neighborhood, where Mr. Gray grew up, residents unanimously agreed with Ms. Mosby’s assertion that Mr. Gray’s death was a homicide. Alethea Booze, 72, said she had witnessed the arrest. “He wasn’t hollering until two officers put that knee in his back and he was screaming,” Ms. Booze said. “Everybody was screaming, ‘Call the ambulance, call the ambulance,’ and the officers didn’t do anything.”

All six officers face administrative hearings led by the police in nearby counties. Four are back at work, a in desk jobs. The Department of Justice is investigating the Baltimore Police Department to determine if it engaged in a pattern of racial discrimination.

Ms. Mosby’s move caused ripples on the presidential campaign trial, as the Republican nominee, Donald J. Trump, who has cast himself as the law-and-order candidate, sharply criticized her, telling reporters, “I think she ought to prosecute herself.”

Ms. Mosby was elected in 2014 on a promise to aggressively prosecute police misconduct; she faces re-election in 2018. But she is under intense pressure from activists who say she has not done enough to prosecute misconduct in less high-profile cases; on Wednesday, she vowed to “fight for a fair and equitable justice system for all, so that whatever happened to Freddie Gray never happens to another person in this community again.”

Mayor Stephanie Rawlings-Blake — who decided not to seek re-election after the unrest — asked residents to be patient as they absorbed the news. On Tuesday night in Philadelphia, the mayor called the roll that resulted in the nomination of Hillary Clinton for president, part of her duties as secretary of the Democratic National Committee.

To Black Lives Matter activists, the outcome was a clear disappointment — though perhaps not a surprise. DeRay Mckesson, a leader of the movement who ran unsuccessfully for mayor of Baltimore, echoed Ms. Mosby’s call for criminal justice reform.

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“The dismissals are a reminder that the laws, practices and policies justify the actions of the police at all costs,” Mr. Mckesson said in a text message. “Freddie Gray should be alive today and someone should be held responsible for his death.”

The officers’ trials opened with a sputter in December, when a jury deadlocked in the case of Officer William G. Porter, who had checked on Mr. Gray during the van ride, but had not belted him in or called medical attention. The mistrial caused delays — Ms. Mosby’s office appealed to Maryland’s highest court in a successful bid to compel Officer Porter to testify against his fellow officers — but the next up, Edward M. Nero, was acquitted in May.

The driver of the van, Officer Caesar R. Goodson Jr., faced the toughest charge — second-degree murder — and was acquitted in June. And Lt. Brian Rice, the highest-ranking officer present for the arrest, was acquitted this month.

Over four trials, prosecutors and defense lawyers argued that Mr. Gray’s injury had occurred in the van, and Judge Williams agreed in his ruling in Lieutenant Rice’s case. But outside the courthouse, in the streets of Mr. Gray’s neighborhood, another theory on his death has thrived: that he was injured by the officers before he got into the van.

The theory is fed by some witness statements and a video showing Mr. Gray being dragged into the van, his legs mostly limp (although he appeared to make some limited motion), that have impressed themselves far deeper into the city’s consciousness than prosecutors’ arguments have.