Najee Rivera admits he panicked on the night two white Philadelphia cops pulled over his motor scooter in El Centro de Oro, a Latino ghetto in the city’s Fairhill section.

“To be honest, I was afraid,” Rivera said. “I saw them get out of their car with nightsticks. I heard one of them call me a spic. I hadn’t done anything wrong, so I took off. I shouldn’t have, but I was scared of them.”

With good reason. A private security camera captured what happened next.

As Rivera puttered along at perhaps 25mph, the police car raced up alongside him. The cop on the passenger side leaned out the window and clocked Rivera on the back on the head with his truncheon, knocking him off his scooter to the pavement. Officers Kevin Robinson and Sean McKnight bounded from the car and began clubbing Rivera as he lay wailing. They hauled him to his feet, slammed him against a building and then drove him back into the sidewalk.

When the beating was over that night, May 29, 2013, Rivera’s wounds required 38 surgical staples to his head and 18 stitches to his face. His nose was broken, an ear was gashed and the orbital socket of his right eye, swollen and plum-colored, was fractured.

The felonious assault on Rivera, then 21, was covered up by Robinson and McKnight with the familiar police-report narrative: The perp was resisting and the cops felt endangered, so they used “necessary force.” The truth came to light in February, when Rivera’s girlfriend, a South Philly nurse named Dina Scannapieco, revealed the smoking-gun security video. The cops were suspended and charged with aggravated assault.

Rivera’s story represents a broader trend in police violence that has been largely overlooked in the recent headline examples, from Cleveland to South Carolina, Baltimore to San Bernardino, Calif. Many of the most appalling examples of police brutality seem to spring from an officer’s rage when a citizen has the audacity to flee. Too many police officers can’t resist a pursuit—on foot or in a patrol car—even though they’ve been schooled repeatedly on the narrow parameters for permissible chases.

Pissing Off Police

“It’s called contempt of cop or POP: pissing off police,” says Geoffrey Alpert, a University of South Carolina criminologist and leading expert on police violence and pursuits. “These guys have a sworn duty to catch the bad guys, and that becomes an overwhelming instinct when someone runs from them. They’re going to try to catch them.”

And when they do, bad things often happen. Nothing seems to transform an otherwise reasonable police officer into a crazed beast faster than someone who flees.

“The psychology of pursuits is a very important factor in so many of these brutality cases, but no one seems to want to pay much attention to it,” says Gregory Gilbertson, a former Atlanta cop who teaches criminal justice at colleges in the Seattle area.

It’s impossible to know how many examples of police violence begin with pursuit rage since the U.S. declines to compile statistics on shootings and assaults by cops. As a result, no one can thoughtfully analyze the genesis of these events, much less make recommendations for how they can be minimized. But a growing record of anecdotal examples—many substantiated by police dash-cams or video shot by witnesses—suggests a pattern.

In one of the more bizarre recent examples, two deputies delivered blows and boots to the head and groin of Frank Pusok, 30, who led law enforcers on a long pursuit by car and on horseback in the Mojave Desert of San Bernardino County, Calif. after Pusok stole a horse. The April 9 beating was captured on video by a news helicopter. Pusok had surrendered and was spread-eagled on his belly when the beating commenced. Each of 10 deputies could not resist getting in a lick or two as they arrived, long after the suspect was handcuffed. They’ve been suspended and may face criminal charges. The county paid Pusok a preemptory settlement of $650,000.

That assault was five days after the shocking shooting death of Walter Scott, 50, who lumbered away from Officer Michael Slager following a run-of-the-mill traffic stop in North Charleston, S.C. Slager fired eight shots, five of which hit Scott in the back. A brief recorded conversation between Slager and a police supervisor after the shooting hints at a crucial component of police pursuits.

"By the time you get home,” the supervisor said, “it would probably be a good idea to kind of jot down your thoughts on what happened. You know, once the adrenaline quits pumping."

"It's pumping," Slager said, laughing nervously.

All About Adrenaline

I asked Sam Walker, an emeritus professor of criminology at the University of Nebraska Omaha and expert on police misconduct, why cops turn psycho during pursuits.

He replied, “Adrenaline, adrenaline and adrenaline, compounded by a failure of the department to adequately train its officers to think about the department’s policies that are designed to curb instincts and impulses and to act rationally and carefully.”

“Police officers engage in these chases then say, It was just my adrenaline," adds Seattle’s Gilbertson. “Please. You are trained to contain your emotions. That is part of your job—to make rational decisions and judgments while under stress.”

“I think most cops view running from them as a crime, even though intellectually they must know it’s not because they have been told in training, or should have been told,” Gilbertson says. “When someone runs, too many officers seem to really believe that they have a right to chase them down and use whatever force is necessary to subdue them.”

In fact, case law and widely accepted police protocols (based on research by South Carolina’s Alpert dating to the 1980s) strictly limit permissible pursuits, both on foot and in vehicles, to those involving suspects in violent felonies or those who may present imminent risk to the public or police.

This is from a primer on auto pursuits by the International Association of Chiefs of Police:

High speed pursuit driving creates enormous civil liability exposures for police officers and agencies and can result in criminal prosecution of police officers as well. Few areas of police work involve higher stakes. The need to conduct some high speed pursuits is obvious to most. Equally obvious is the need to protect the public (and police officers themselves) from unnecessary risks created by indiscriminate high speed chases.

Gilbertson says most pursuits are “totally unwarranted” and “indicate a lack of good order, discipline and supervision in the field.”

“We’ve known about car-pursuit dangers for decades now, and foot pursuits may be even more dangerous,” he says. “And yet most police pursuits still start with minor infractions or an officer’s suspicions. It’s crazy.”

“None of these cases ever begins with an officer saying, I’m going to go out and kill somebody,” Alpert told me. “But the process is the same. You get ramped up, you get excited, and you get yourself deeply invested emotionally in a pursuit. You think what you’re doing is right, you believe what you’re doing is right, but it turns out not to be right.”

‘Grumpy and Frustrated’

Cops involved in a chase can feel a sense of “righteous indignation,” says Rodger Broome, who spent 17 years in law enforcement in Utah and now teaches at Utah Valley State University in Orem and volunteers as a reserve officer.

“You might be thinking, This person is going to kill me and take me away from family," Broome says. “So, yes, some of us do tend to take it personally, like, You’re not going to make an orphan out of my kid."

Certain officers can’t resist acting out with physical retribution.

“Sure, you’re probably a little grumpy and frustrated with the guy when you catch up to him,” says Gilbertson. “An officer will personalize or internalize the fact that they were running from me. Well, it’s not personal. They are running from the law, running from a uniform, for whatever reasons they might have. We might not think it’s a legitimate reason, but that’s not necessarily our business. They have their reasons. That’s their prerogative.”

Whether it was adrenaline, righteous indignation or group-think, police in Cleveland personalized a chase on Nov. 29, 2012. It begin with an unconfirmed report that someone had fired a shot near police headquarters in the city.

Officers “believed” the shot had come from a car driven by Timothy Russell, 43, according to the Cleveland Plain Dealer. A long, perilous police pursuit began, reaching speeds of 110mph. By the time the chase ended 25 minutes later, 62 police cars and 104 of the 277 Cleveland cops on duty that night had joined in.

When Russell finally stopped, 13 officers fired a fusillade of 137 bullets into his car. One officer, Michael Brelo, climbed atop the car’s hood and fired 49 shots. Russell was hit 23 times and his passenger, Malissa Williams, 24 times. Both were killed. No weapon was found in the car, and no evidence linked Russell to the report of the shot.

Brelo, 31, a five-year police veteran and an ex-Marine who had served in Iraq, went on trial for manslaughter this month. He was awaiting a verdict this week. His union president has called him a hero. Meanwhile, Cleveland has agreed to pay the victims’ families $3 million.

Foot Chases: Same Story

Three other recent controversial police-custody deaths involved brief foot pursuits:

Last Aug. 9, Michael Brown, 18, was shot and killed by Ferguson, Mo., Officer Darren Wilson after the teenager moved away from the officer following a tussle. The case inspired rioting. Wilson was cleared of wrongdoing.

On April 2 this year, Eric Harris, 44, was shot and killed by a Tulsa, Okla., reserve deputy, Robert Bates, 73. Harris fled during an u ndercover gun buy . Bates, who rolled up after Harris was caught, claimed he confused his handgun and his Taser. He faces manslaughter charges.

Harris fled On April 12, Freddie Gray was chased and arrested by Baltimore police after he made suspicious “eye contact” with bicycle officers at 8:40am. A charging document said Gray "fled unprovoked upon noticing police presence." He was bundled into the back of a transport van, where he apparently suffered a fatal neck injury and died a week later. Six police officers are charged in his death, which prompted rioting in Baltimore.

“Everyone asks the same question about the Freddie Gray case,” says Gregory Gilbertson. “’Why was he running? If he’s so innocent, what made him run?’ Well, I don’t care what Freddie Gray was doing and why he ran…Just because they run from you doesn’t justify you chasing them. Simply looking at a police officer and taking off running is not a crime. Is it suspicious? Yes. But it’s not a crime….Maybe they’re afraid and just want to get away. We should always remember that a certain segment of the American population has never had a good experience with a police officer.”

Gilbertson says young, inexperienced cops are responsible for most reckless chases. He was once one of them, chasing teenagers around an Atlanta housing project.

“I finally figured it out after a while,” he said, “that it was a whole lot easier to talk to people than to chase ‘em down and fight ‘em.”

According to standard police policy, a supervisor—older and wiser, in theory—is supposed to call off dangerous pursuits for minor infractions.

“I don’t want to say it’s entirely a function of age,” says South Carolina’s Geoffrey Alpert. “But age can explain some of it. Young guys are full of spit and vinegar, they feel 10 feet tall, and they want to be the good guy who catches the bad guy…But you’ve got to disassociate yourself. It’s not a personal challenge, though it’s certainly not hard to take it personally, especially in foot pursuits.”

Acute anger “is not only a possibility but a probability” in pursuits, particularly foot chases, says Utah’s Rodger Broome.

“It’s not exactly a Darwinian thing like a cheetah or some other predator chasing prey,” he adds. “But for an officer in a pursuit, fear and anger can become very blended. You might be feeling some automatic disdain for criminals because they are our enemies. And then you might be thinking, You put me through all this for an expired driver’s license?"

‘Why Were You So Angry?’

Back in Philadelphia, Najee Rivera is asking his own version of that question. He was properly licensed and riding his scooter legally when officers Robinson and McKnight pulled him over. Why did they do it?

“He’s Hispanic, and the cops were white,” says his girlfriend, Dina Scannapieco. “I’m white, so I’m embarrassed to say this, but it seems racial. They called him an effin’ spic. Plus, there are a lot of guys on dirt bikes that run from the police in that neighborhood. He was on a scooter. I think they figured, Oh, we can catch this one."

Rivera says his right eye is still sometimes blurry, and he pops Motrin to dull a debilitating recurring migraine. He lost his job as a maintenance worker at Children’s Hospital of Philadelphia.

“Before this happened, my life was great, my life was wonderful,” he told me. “I was working, I had my family. But ever since that happened, I’m a different person. I feel like I’m damaged.”

Philadelphia police gave Scannapieco short shrift when she filed a brutality complaint. For 18 months, Rivera’s lawyer sat on the surveillance video Scannapieco had unearthed by going door to door near the scene of the beating. When the footage finally was revealed, the city suspended and charged the cops and settled with Rivera for $200,000. He was left with half that after the lawyer took his $60,000 fee and Rivera paid medical bills.

“It’s nothing,” says Scannapieco. “He was making $20 an hour at the hospital. That’s a career. That’s not McDonald’s. And what’s he got now? He’s damaged forever.”

Surprisingly, Rivera told me he’d like to speak with the cops who beat him.

“There’s a question I really want to ask them,” he says. “Why did you get so violent? At the end of the day, I didn’t cause you any harm. Why were you so angry at me?”