BALTIMORE -- A judge explained why he found Officer Caesar Goodson not guilty in the death of Freddie Gray, a 25-year-old black man whose neck was broken on the way to the station: He didn't see any evidence of a crime.

Baltimore Judge Barry Williams ruled Thursday that the state failed to prove the police driver committed murder, manslaughter, assault, reckless endangerment or misconduct in office.

"There has been no evidence that this defendant intended for a crime to happen," Williams said. "The state had a duty to show the defendant corruptly failed in his duty, not just made a mistake."

Goodson's acquittal is the worst blow yet to efforts to hold police accountable in a case that triggered riots in the troubled city. The state's third failure to convict also casts doubt on whether any of the six indicted officers will be found guilty.

Baltimore Police Officer Caesar Goodson leaves the courthouse at the end of day four of the murder trial of black detainee Freddie Gray in Baltimore, Maryland, on June 14, 2016. Reuters/Bryan Woolston

Baltimore's police union president, Gene Ryan, called on State's Attorney Marilyn Mosby to reconsider her "malicious prosecution," since he's certain the remaining officers also will be found without guilt.

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Doug Colbert, a law professor at the University of Maryland, has attended all three trials.

"It means that convicting a police officer in this country is an exceedingly rare event," Colbert told CBS News Justice and Homeland Security correspondent Jeff Pegues. "It also means there is a great deal of empathy and understanding of the police officer and much less of the people who find themselves suffering injuries and in this case losing their lives."

Throughout the legal proceedings, prosecutors' tactics have been maligned, CBS News correspondent Jeff Pegues reports.

In December, Officer William Porter walked free after a hung jury. Last month in another bench trial, Williams found Officer Edward Nero not guilty. Both officers faced lesser charges than Goodson.

At the start of Goodson's trial earlier this month, Williams scolded prosecutors for failing to turn evidence over to the defense about another prisoner in the transport van with Gray.

On Thursday, Mayor Stephanie Rawlings-Blake said in a statement posted to Twitter that Goodson still faces a police administrative review.

"We once again ask the citizens to be patient and to allow the entire process to come to a conclusion," she said.

Her likely successor, State Sen. Catherine Pugh, pleaded for patience.

"Although people may disagree with the verdict, it is important to respect each other and to respect our neighborhoods and our communities," Pugh said.

The case became a rallying cry for the growing Black Lives Matter movement and sparked more outrage nationwide over how black people are treated by police and the criminal justice system.

But the case hasn't fit quite so neatly into the American narrative of white authorities imposing justice unfairly on black people. In this case, the defendant, trial judge, state's attorney and mayor are African-American; at the time of Gray's death, so was the police chief.

After the verdict, Black Lives Matter activist and Baltimore native DeRay Mckesson aimed his criticism at the entire system.

"Today is a reminder that there is a set of laws, policies and police union contracts across the country that will protect any form of police behavior," he said.

Goodson was the only one of the six officers charged in Gray's death to be accused of murder.

Mosby appeared crestfallen as she hurried out of the courthouse and sped away in a black SUV. When she confidently announced the charges last year, she reflected the anger of many people who saw the case as open-and-shut.

State's Attorney Marilyn Mosby, center, departs the courthouse in Baltimore, Maryland, on June 23, 2016, after Officer Caesar Goodson was found not guilty of all charges related to the death of Freddie Gray. Reuters/Bryan Woolston

Many still feel that way, McKesson said.

"It's a matter of fact that he was alive when he got in the van. It's a matter of fact that his spine was severed," he said. "Six people contributed to that."

Prosecutors said Goodson was criminally negligent because he didn't buckle Gray into a seat belt to keep him from slamming into the walls of the van's metal compartment after officers bound his hands and feet.

The state also said Goodson exhibited a disregard for Gray's life when he chose not to call a medic after Gray repeatedly indicated that he wanted to go to a hospital.

But Goodson wouldn't talk to investigators or take the stand at trial, and there were no witnesses or cameras to show what happened inside his van, leaving the state with slim evidence of intent to harm.

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

"The failure to seatbelt may have been a mistake or it may have been bad judgment, but without showing more," he added, "... the state has failed to meet its burden to show that the actions of the defendant rose above mere civil negligence."

The judge said Goodson wouldn't have been able to determine whether Gray was injured between the second and fifth stops on the 45-minute trip, because only after the van finally arrived at the Western District station house did officers observe Gray with blood and saliva on his face, and evidence that he'd become incontinent, Williams 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?"

The judge also dismissed the "rough ride" theory, which he called the centerpiece of the state's case. Proving it required evidence of actions and intent, and the state provided neither, he said.

"Seemingly the state wants this court to assume, because Mr. Gray was injured, that the defendant gave him a rough ride," Williams said, adding, "there is insufficient evidence to prove the defendant gave Mr. Gray a rough ride."

Gray was arrested April 12, 2015, after running from an officer on bike patrol outside a public housing project. A neighbor's video showed him handcuffed behind his back and hoisted into Goodson's van, which made six stops on his odyssey to the nearby station house.

The violence following Gray's death a week later forced Maryland to bring in the National Guard. Rawlings-Blake fired the police commissioner and then abandoned her re-election campaign. The Justice Department is formally investigating allegations of widespread police abuse.

Baltimore defense attorney Warren Alperstein, who isn't involved in the case, said the state now faces "significant issues moving forward" against the other officers.

"The state's attorney's office has committed to seeking justice," he said. "On the other hand, does there come a time when you have to cut your losses?"

New York-based defense attorney Barry Slotnick, who successfully defended "subway vigilante" Bernard Goetz in a 1984 subway shooting, said he believes "The DA's office is stuck."

"If they withdrew the charges, the community would be outraged," he said.