Prosecutor Marilyn Mosby had stunned the city and became a national figure by filing charges against six officers just days after Gray's death from a broken neck suffered in a police van sparked protests and rioting in April 2015.
The death of the 25-year-old was one in a series of high-profile deaths of black suspects at the hands of U.S. police that stoked a national debate on police tactics and the treatment of minorities. It fueled the rise of the Black Lives Matter movement, which protests excessive police use of force against minorities.
The decision to drop charges against the three remaining officers facing trial came on the day of a pretrial hearing for Officer Garrett Miller, whose trial was to start on Thursday in Baltimore City Circuit Court.
At a news conference held before a mural in Gray's neighborhood memorializing him, an irate Mosby said that individual police officers had tried to thwart her investigation.
The interference included officers who were witnesses investigating the case and key questions not being asked during interrogations. A police counter-investigation aimed at disproving the prosecution's case also failed to execute search warrants, Mosby said.
"We've all been witness to an inherent bias that is a direct result of when police police themselves," she said. "Police investigating police, whether they are friends or merely their colleagues, was problematic."
A spokeswoman for the Baltimore police had no immediate comment on Mosby's remarks.
Prosecutors last week failed in their fourth attempt to secure a conviction against a police officer in the case. Baltimore's police union had called on prosecutors to drop the charges against the remaining officers.
Judge Barry Williams acquitted three officers in bench trials, and the trial of a fourth officer ended in a deadlocked jury.
Reacting to the decision, Republican presidential nominee Donald Trump said Mosby had made a bad call in prosecuting the officers.
"It was disgraceful what she did and the way she did it," he told reporters in Florida.
Over the course of the trials, prosecutors said the officers gave Gray a "rough ride," and failed to secure Gray in the back of the van as outlined in department protocol or seek medical assistance.
But defense lawyers said that officers had the discretion on whether to seatbelt detainees and it was unclear when Gray suffered his fatal neck injury.
The Gray case represented one of the first high-profile cases in which a prosecutor chose to go after officers involved in a black suspect's death, after grand juries declined to charge officers involved in the shooting of 18-year-old Michael Brown in Ferguson, Missouri, and in the choking death of Eric Garner, 43, in New York.
More recently, federal prosecutors launched a civil rights investigation into the shooting of a black man by police in Baton Rouge, Louisiana, and Minnesota officials began a probe into a fatal shooting of a black motorist outside St. Paul. Both killings occurred earlier this month and set off fresh protests.
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