It was one of the most closely watched cases in recent years of a black man dying after being in police custody.
With Baltimore reeling from demonstrations that sometimes had become violent,
"I heard your call for 'no justice, no peace.' Your peace is sincerely needed as I work to deliver justice on behalf of this young man," Mosby said just over a year ago on national television during a news conference.
Civil rights activists praised the prosecutor for swift and comprehensive action.
"It's not looking great. These are weak cases," said Steve Levin, a former federal prosecutor in Maryland who has represented police officers in criminal trials. "It's easy to bring charges against officers. But it's much harder to prove them."
Prosecutors said Nero -- one of three officers on bike patrol who chased and arrested Gray on April 12, 2015 -- assaulted Gray by detaining him without justification. The reckless endangerment charge was from Nero putting Gray in a police van without buckling the seat belt.
In his ruling Monday, Baltimore Circuit Judge Barry Williams said there were "no credible facts" to show that Nero was directly involved in Gray's arrest. Testimony suggested that Nero played a minimal role in putting Gray in the van and that his actions were in line with his training, the judge said.
The ruling was a second blow to the prosecution. The trial of another officer,
Legal experts say the cases against many of the officers involved are weak.
"To prove these officers are guilty for many of these charges, like involuntary manslaughter, the prosecution has to convince a jury or judge that they did not only fail to do their duty but that they also put Gray at a high risk for dying," said David Jaros, a law professor at the University of Baltimore.
He said the strongest case is likely to be against Caesar Goodson Jr., who drove the van in which Gray suffered spinal injuries that resulted in his death a week after his arrest.
He could face up to a 30-year sentence if found guilty of second-degree murder, the most serious charge against any the officers. His trial is scheduled to start June 6.
On Monday, national and local activists lamented the Nero case but said they still hoped for convictions in future trials.
"The Nero verdict is a reminder that we must continue to push for policies and laws related to the police department that explicitly call for the preservation of life and that have clear lines of accountability," said DeRay Mckesson, a Black Lives Matter activist and former candidate for Baltimore mayor. He called the verdict "disappointing but not unexpected," adding that he was "reminded that this is one of six trials as we seek accountability for the death of Freddie Gray."
A statement from the NAACP echoed similar views. "We await justice for Freddie Gray," it said. "We respect the legal process and pray that the family of Freddie Gray will receive justice for his tragic death."
After several high-profile incidents in which officers were not indicted -- including the deaths of 18-year-old Michael Brown in Ferguson, Mo., and 43-year-old Eric Garner in Staten Island, N.Y. -- activists had cheered the quick action of state officials against the Baltimore police officers.
But now some activists and legal experts are suggesting that Mosby acted too quickly, questioning whether a May 21, 2015, grand jury indictment of officers left enough time for investigation.
Mosby is under a gag order while other cases proceed, and her office did reply to a request for comment.
Indictments in some other high-profile cases took months or longer, not weeks. In June, a state grand jury indicted former North Charleston, S.C., police Officer Michael Slager on murder charges in the death of 50-year-old Walter Scott, whom Slager shot two months earlier as Scott ran away after a traffic stop. This month, Slager was indicted on federal charges.
In another high-profile case, Chicago Officer Jason Van Dyke was indicted in December on first-degree murder charges in the shooting of 17-year-old Laquan McDonald in October 2014.
Peter Moskos, a former Baltimore police officer and associate professor at John Jay College of Criminal Justice in New York, said the Baltimore cases have become a referendum on how police interact with racial minorities and residents of poor neighborhoods. Gray lived in the Gilmor Homes public housing complex, near where he first encountered Nero and Brian Rice, another officer on bike patrol, who is due for trial on July 5.
"You're rarely going to find moral justice in criminal prosecution" of police officers, he said. "But people will look for it, and they might be let down."