BALTIMORE – A judge acquitted a police driver of all charges on Thursday in the death of 25-year-old arrested Freddie Gray, whose broken neck on the way to the station set off Baltimore’s worst riots in decades.

Six officers were charged in Gray’s death, but only Officer Caesar Goodson was accused of “depraved heart” murder. Gray was fatally injured after officers bound his hands and feet and Goodson left him unprotected by a seat belt that prosecutors say would have kept him from slamming into the van’s metal walls.

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Baltimore Circuit Judge Barry Williams also found Goodson not guilty of manslaughter, assault, misconduct in office, and reckless endangerment after five days of testimony in the non-jury trial.

Williams said the state failed to show that Goodson knew he’d harm Gray by leaving him unbuckled, or that he was aware of the injury.

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“The state failed to prove the defendant knew or should have known that Mr. Gray needed medical care,” the judge said.

“Unlike in a shooting or a stabbing, or a car accident, this injury manifests itself internally,” Williams said, citing conflicting testimony from medical experts. “If the doctors weren’t clear, how would a person without medical training know?”

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The judge also said the state failed to prove Goodson gave Gray a “rough ride.”

“The state had a duty to show the defendant corruptly failed in his duty, not just that he made a mistake,” the judge said.

Outside the courthouse, a small group of protesters chanted: “We can’t stop. We won’t stop, till killer cops are in cell blocks.”

Protests and rioting after Gray’s death on April 19, 2015 set the city on fire, forcing Maryland to bring in the National Guard. The unrest forced the city’s mayor to abandon her re-election campaign, and the Department of Justice opened an investigation into allegations of widespread police abuse.

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WATCH: Freddie Gray protesters angered over Baltimore cop’s acquittal

2:32 Freddie Gray protesters angered over Baltimore cop’s acquittal Freddie Gray protesters angered over Baltimore cop’s acquittal

The Democratic nominee to become Baltimore’s next mayor, State Sen. Catherine Pugh, issued a statement pleading for patience.

“Protests are a vital part of democracy, but to destroy the homes and businesses many people have worked very hard to build is unacceptable. Although people may disagree with the verdict, it is important to respect each other and to respect our neighbourhoods and our communities,” Pugh said.

Prosecutors said Goodson was criminally negligent when he failed to buckle Gray into a seat belt or call for medical aid after Gray indicated that he wanted to go to a hospital. But Goodson wouldn’t talk to investigators or take the stand at trial, leaving the state with slim evidence of intent to harm.

The acquittal of Goodson, 46, is perhaps the most significant blow to State Attorney Marilyn Mosby’s efforts to hold police accountable for Gray’s death.

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READ MORE: Baltimore police leader acknowledges flaws exposed by Freddie Gray case

Last month, the same judge acquitted Officer Edward Nero of misdemeanour charges, and in December, he declared a mistrial after a jury failed to agree on manslaughter and other charges against Officer William Porter.

Porter faces a retrial in September, and three other officers have yet to be tried.

Grey was arrested April 12 after running from an officer on bike patrol outside a public housing project not far from the Western District station house. A neighbour’s video showed him handcuffed behind his back and hoisted into Goodson’s van.

The van made a total of six stops that day, and Gray was unresponsive on arrival at the station house 45 minutes later. Prosecutors said Goodson was there throughout and checked on Gray during the third and fourth stops, so he should have known Gray was in distress. They said his failure to call a medic amounts to murder.

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A prosecution expert testified that Gray could not possibly have broken his own neck. Prosecutors said the injury happened somewhere between the second and fourth stops, when Goodson and Porter lifted Gray off the floor.

Porter testified that Gray was lethargic, but could breathe and speak, and didn’t seem injured. Prosecutors countered that the initial injury became critical as the trip continued.

Second-degree “depraved heart” murder, which carries up to 30 years in jail, would mean that Goodson was so negligent in his inaction that he cast aside any consideration for Gray’s life.

During opening statements, prosecutors for the first time accused Goodson of giving Gray a “rough ride,” intentionally leaving him unbuckled “to bounce him around in the back of the van.”

But by closing arguments, they all but abandoned the theory, saying Goodson’s failure to belt Gray in under the circumstances was sufficient to prove the intent necessary for a murder conviction.

“Officer Goodson never calls a medic, he never takes Freddie Gray to the hospital,” said Deputy State’s Attorney Jan Bledsoe. “He has breached his duty, and because of that breach Freddie Gray’s life was shortened.”

READ MORE: ‘Rough ride’ theory offered by prosecutors in trial of cop accused in death of Freddie Gray

The judge seemed skeptical, peppering prosecutors with questions and asking what evidence they had supporting the “rough ride” theory. What if Gray had emerged from the van unhurt, despite being unbuckled, and was found to be falsely claiming injury in order to avoid jail?

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Chief Deputy State’s Attorney Michael Schatzow said the failure to belt him in would still be a crime, although a difficult one to prosecute.

Goodson’s attorney Matthew Fraling fiercely rejected the allegations, telling the judge that Goodson was a “gentle” officer who didn’t buckle him in because Gray was exhibiting “violent and erratic” behaviour, citing witness testimony that he was making the wagon shake back and forth by kicking and flailing inside.

Fraling also said Gray said yes when Porter asked if he wanted to go to a hospital only because Gray hoped to avoid jail.

“They have failed to cobble together any type of case with reasonable inferences, let alone evidence,” he said. “The mere fact that harm resulted doesn’t mean the Officer Goodson’s conduct is the cause of that harm.”

Goodson’s acquittal may impact the remaining cases. Officer Garrett Miller and Lt. Brian Rice are scheduled to stand trial in July on charges of assault, reckless endangerment and misconduct in office; Rice also faces a manslaughter charge.

All the officers but Goodson have filed defamation lawsuits against Mosby and Maj. Sam Cogen of the sheriff’s office, who signed the charging documents. The officers claim the criminal charges amounted to false and damaging information.