Captured by a video camera on one of the officer’s helmets, the slaying by Albuquerque police of a homeless camper in the foothills outside the city two weeks ago apparently leaves room for interpretation. The city says it was justifiable self-defence. To others it looked like extra-judicial execution.

The grisly clip, which is still viewable for anyone with a strong stomach, has an awful power all of its own. But it has also become the tragic totem of a wave of civic anger directed at the city’s heavily militarised police department that on Sunday erupted into hours of unrest across the downtown area, leading to serial arrests and a plea for calm from the city’s mayor who said his streets had turned to “mayhem”.

For James Boyd, the fury comes too late. With a long history of mental problems and episodes of violence, he was challenged by three officers on 16 March for camping in an unauthorised area near the city limits. They woke him from sleeping. A three-hour stand-off ensued until, at dusk, he is heard telling the officers he is done arguing and was “going to walk” with them. That is where the episode might have ended.

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Shape Created with Sketch. The Albuquerque shooting Show all 4 left Created with Sketch. right Created with Sketch. Shape Created with Sketch. The Albuquerque shooting 1/4 Clip 1 APD officers are seen confronting the homeless man 2/4 Clip 2 As he turns away from the officers, shots are fired 3/4 Clip 3 Mr Boyd is seen lying on the ground 4/4 Clip 4 A shot is then fired as he lies immobilised 1/4 Clip 1 APD officers are seen confronting the homeless man 2/4 Clip 2 As he turns away from the officers, shots are fired 3/4 Clip 3 Mr Boyd is seen lying on the ground 4/4 Clip 4 A shot is then fired as he lies immobilised

Instead, as Mr Boyd gathers up his things, one of the officers shouts “Do it”. A flash-bang device is fired at his feet. A startled Mr Boyd drops his bags and seems to take out a knife. He turns away from the officers, two of whom shoot multiple live rounds into his back. Mr Boyd falls, a dog is loosed to check he has been immobilised, the officers approach and cuff his wrists.

He was pronounced dead later in a city hospital.

The killing of Mr Boyd, who was 38, prompted more than the usual handwringing because it seemed less an isolated incident than a confirmation of a pattern.

Since 2010, the Albuquerque Police Department (APD) has been involved in 37 shootings, 23 of them fatal. According to the group ProgressNow, its officers shot more people than the NYPD over the same period did in New York – a city 16 times larger.

While Police Chief Gorden Eden said in a later press conference that the actions of the three officers had been justified because Mr Boyd had represented a “direct threat” to their safety, the killing is now being officially investigated by the FBI.

Even before the slaying, the US Justice Department had, for over a year, been investigating wider claims of human rights and other abuses by the APD. Neither probe is yet complete.

The Boyd slaying appears simply to have been one too many.

Or maybe it has had such an impact because it can so easily be viewed. But the city might have guessed at the trouble that was coming when a YouTube video emerged vowing retaliation for the homeless man’s death. Taking credit for the threat was Anonymous, the underground collective that hacks into and debilitates the web sites of those it opposes.

Sure enough, the APD confirmed early Sunday that its website had been broken into and crippled for hours. At about the same time, the protesters, several hundred of them, were gathering in the centre of Albuquerque, New Mexico’s largest city. Through the daylight hours they repeatedly marched through downtown, blocking traffic, chanting and brandishing banners demanding that the APD be cleaned out and Chief Eden be forced to resign. For the most part, the marches were peaceful if disruptive.

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But by Sunday evening, the mood had changed.

Witnesses reported hours of clashes between the protesters and police in riot gear, some on horseback. News video shows tear gas canisters being repeatedly released into the crowds. In one of the more serious encounters, protesters trapped a squad car and tried to smash its windows with bricks and rocks.

“It has reached a boiling point, and people just can’t take it any more,” said Alexander Siderits, 23, who was among those marching on Sunday, adding that many of the city’s residents were just “fed up” with the police and their record of fatal shootings.

Justin Elder, who followed the march in the safety of his car, held a sign reading, “APD: Dressed To Kill”. He explained: “That’s what this police force is about.”

While the city was reported to be calm yesterday, on Sunday Mayor Richard Berry said its downtown area had been reduced to “mayhem”, blaming outside elements for exploiting the situation.

“We respected their rights to protest obviously,” he said, “but what it appears we have at this time is individuals who weren’t connected necessarily with the original protest. They’ve taken it far beyond a normal protest.”

Still unanswered is why two of the officers used lethal firearms to take Mr Boyd down instead of stunning him with the taser guns they were also reportedly carrying. They have been placed on paid administrative leave.

The broader debate is about the APD navigating the thin line between reasonable and unreasonable force to maintain order, and the apparent unwillingness of the city’s leaders to make sure it isn’t crossed.

“I was a police officer for a decade,” says Patrick Davis, executive director of ProgressNow New Mexico. “The over-militarized approach to law enforcement is having a very real effect on people’s lives here in New Mexico and our leaders who should be taking real action seem to be taking it all in stride.”