This dangerous practice allows police to injure suspects without ever laying a hand on them

The practice of giving suspects "rough rides" in the back of police vans has become a tradition among officers who wish to retaliate against "mouthy" offenders without ever being accused of physically assaulting them, the New York Times reports.

An investigation is under way to determine exactly how 25-year-old Freddie Gray sustained his fatal injuries while in the back of a Baltimore PD police van last month.

Gray was arrested in April on suspicion of possessing a switchblade and died a week later after suffering a severe spinal cord injury sustained during a 45-minute ride in a police van. All six officers involved in his arrest have since been indicted on criminal charges, and the prosecutor responsible for bringing them says Gray's knife was, in fact, legal.

A working theory is that the driver of the police van, knowing Gray had not been strapped in, intentionally gave Gray a rough ride to get back at him for acting "irate."

"It's sort of a retaliatory gesture," Robert W. Klotz, a police-procedures expert and former deputy chief of police of the Metropolitan Police Department in Washington, told the Times. "It's one of those nebulous type of things where the individual feels they've been subjected to it because they've been mouthy. The officers say they have no intent in doing anything. It winds up in a he said-she said situation."

In Baltimore and other cities, police have used "rough rides" as payback in the past, according to the LA Times. Police departments across the country have reportedly paid thousands in legal settlements related to injuries sustained during these "nickel rides," as they are called in Philadelphia, referring to the old-time amusement park rides that cost five cents.

"Very fast, wide turns, braking short - they were doing everything they could to make the ride as bumpy and chaotic as possible," Christine Abbott, a 27-year-old librarian at Johns Hopkins University who is suing Baltimore for injuries sustained during her rough ride, told the LA Times. "I was just sliding around in there."

Another Baltimore man won a $7.4 million lawsuit against the city after a rough ride in a police van left him paralyzed in 2005, according to the LA Times. Philadelphia resident James McKenna was awarded $490,000 after a wild ride left him with a broken neck in 2011.

"It was a way to punish them without really putting your hands on them," Geoffrey Alpert, a professor at the University of South Carolina and an expert in police practices, told the LA Times.

An injury Gray sustained to his head matched one of the bolts in the back of the police van he was riding in, according to reports released by Baltimore's medical examiner last Thursday. The examiner, who ruled Gray's death a homicide, believes Gray's fatal neck injury "was caused when he slammed into the back of the police transport van" rather than during his actual arrest.

The fact that Gray sustained his injuries in the van, however, does not necessarily mean he was given a rough ride intentionally by the driver.

"I never saw it, but I've heard about it," Bernard K. Melekian, the undersheriff of Santa Barbara County, California, and a former director of the Justice Department's community-oriented policing office, told the Times. "My sense was that that kind of behavior had long been gone."

Baltimore police have "acknowledged that officers violated protocol by not buckling in Gray and providing medical help when he asked for it," The Baltimore Sun reported. But they continue to deny that Gray was given a rough ride on purpose.