The remaining charges against three Baltimore police officers awaiting trial over the death of black man Freddie Gray have been dropped.
Prosecutors blamed police for a biased investigation that failed to yield any convictions.
Mr Gray was 25 when his neck was broken while he was handcuffed and shackled but left unrestrained in the back of a police van in April 2015.
His death added fuel to the growing Black Lives Matter movement, setting off massive protests in Baltimore and leading to the worst riots the city had seen in decades.
The decision by prosecutors came after a judge acquitted three of the six officers charged in the case, including the van driver who the state considered the most responsible and another officer who was the highest-ranking of the group.
A mistrial was declared for a fourth officer when a jury reached deadlock.
The case led the police department to overhaul its use of force policy, and all officers will soon be equipped with body-worn cameras.
The US Justice Department has also launched an investigation into allegations of widespread abuse and unlawful arrests by the department.
The officers have sued State's Attorney Marilyn Mosby, saying she intentionally filed false charges against them.
Mr Gray's family received a settlement of $6.4m from the city.
A defiant Ms Mosby held a news conference shortly after the announcement, saying there was "a reluctance" and "an obvious bias" among some officers investigating the case.
"We do not believe Freddie Gray killed himself," she said, standing in the neighbourhood where Mr Gray was arrested, a mural of him on a wall over her shoulder.
"We stand by the medical examiner's determination that Freddie Gray's death was a homicide."
She walked up to the podium as people chanted "We're with you" and her remarks were punctuated by shouts of support.
Mr Gray's mother, Gloria Darden, stood by Ms Mosby, saying police lied.
"I know they lied, and they killed him," she said.
Attorneys for the officers also planned a news conference.
The day started with a pretrial hearing for Officer Garrett Miller - who had faced assault, misconduct in office and reckless endangerment charges.
But chief deputy state's attorney Michael Schatzow told the judge that prosecutors were dropping the charges against Officer Miller and the rest of the officers.
Prosecutors had said Mr Gray was arrested illegally after he ran away from a bike patrol officer and the officers failed to buckle him into a seat belt or call a medic when he indicated he wanted to go to a hospital.
Ms Mosby wasted little time in announcing charges after Mr Gray's death - one day after receiving the police department's investigation while a tense city was still under curfew - and she did not shy from the spotlight.
She posed for magazine photos, sat for TV interviews and even appeared on stage at a Prince concert in Mr Gray's honour.
The city's troubles forced Mayor Stephanie Rawlings-Blake, who has taken a prominent role on the podium at the Democratic National Convention, to fire her reform-minded police chief and abandon her re-election campaign.
Many feared that the acquittals could prompt more protests and unrest, but they never happened and the streets appeared mostly calm on Wednesday.
The Gray case has not fit quite so neatly into the narrative of white authorities imposing unfair justice on minorities.
Three of the officers who were charged are white and three are black. The victim, judge, top prosecutor and mayor are African-American, as was the police chief at the time of Mr Gray's death.
No reputations hinged on the case's outcome as much as Ms Mosby and her husband, Nick Mosby, a councillor for Baltimore's west side who announced his mayoral candidacy shortly after Ms Rawlings-Blake pulled out.
Marilyn Mosby said: "We've all bore witness to an inherent bias that is a direct result of when police police themselves."