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BALTIMORE — No video captured what happened to Freddie Gray inside the police van where officers heaved him into a metal compartment after pinning him to a sidewalk. The cause of his fatal spine injury has not been revealed.

But a troubling detail emerged as hundreds of protesters converged on City Hall again Thursday: He was not only handcuffed and put in leg irons, but left without a seat belt during his trip to the station, a police union's lawyer said.

Unbelted detainees have been paralyzed and even killed by rough rides in what used to be called "paddy wagons." It even has a name: "nickel rides," referring to cheap amusement park thrills.

Police brutality against prisoners being transported was addressed just six months ago in a plan released by Baltimore officials to reduce this misconduct. Department rules updated nine days before Gray's arrest clearly state that all detainees shall be strapped in by seat belts or "other authorized restraining devices" for their own safety after arrest.

Gray was not belted in, said attorney Michael Davey, who represents at least one of the officers under investigation.

But he took issue with the rules.

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"Policy is policy, practice is something else," particularly if a prisoner is combative, Davey told The Associated Press. "It is not always possible or safe for officers to enter the rear of those transport vans that are very small, and this one was very small."

Assistant Police Commissioner Jerry Rodriguez said Gray was secured by "leg irons" after he became agitated during the trip, but the department hasn't said whether he was buckled in with a seat belt.

The Gray family's lawyer, Billy Murphy, said "his spine was 80 percent severed" while in custody. It's not clear whether he was injured by officers in the street or while being carried alone in the van's compartment.

But if it happened on the way to the station, it wouldn't be the first such injury in Baltimore: Dondi Johnson died of a fractured spine in 2005 after he was arrested for urinating in public and transported without a seat belt, with his hands cuffed behind his back.

"We argued they gave him what we call a 'rough ride,'" at high speed with hard cornering, said Attorney Kerry D. Staton. "He was thrown from one seat into the opposite wall, and that's how he broke his neck."

Staton obtained a $7.4 million judgment for the family, later reduced to the legal cap of $200,000.

It also has happened in Philadelphia, where police in 2001 barred transportation of prisoners without padding or belts after The Philadelphia Inquirer reported that the city had paid $2.3 million to settle lawsuits over intentionally rough rides, which permanently paralyzed two people.

Gray fled on foot and was captured on April 12 after an officer "made eye contact" with him outside a public housing complex, police said. Videos show Gray screaming on the ground before being dragged, his legs limp, into a van. Witnesses said he was crying out in pain.

Kevin Moore, a friend of Freddie Gray's who recorded video of his arrest, told The Baltimore Sun that police had Gray's legs bent "like he was a crab or a piece of origami."

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Protesters have made their way from City Hall to the Western Dist. Police Station. Officers standing behind barricade pic.twitter.com/kXPZGM1SeN — Karen Campbell (@KarenCampbellTV) April 23, 2015

— The Associated Press