When five police cruisers entered a mini-mall parking lot earlier this week in Rialto, Calif., it might have signaled trouble.

Three men were handcuffed – two Latinos and an African-American. For about 20 minutes, the police questioned the three men as a crowd gathered. At last, everyone was let go.

There could be a number of reasons why the tension-filled exchange did not descend into violence. But Vernon Tucker, the head of security at the mini-mall, is convinced that the body cameras the police were wearing were a factor.

“I do think it was the cameras that kept this from going south,” says Mr. Tucker, who has worked at the mini-mall for five years and says the cameras have helped out in numerous, potentially explosive episodes across the city. “There’s no way of knowing in each individual case, of course, but my take is that they have helped over all.”

In the aftermath of the police shooting of black teenager Michael Brown in Ferguson, Mo., in August, police bodycams were touted as a potentially effective way both to rein in police violence and to offer greater clarity about exactly what happens during police encounters with citizens. In many ways, Rialto has become the country's most compelling test case of this idea.

Body cameras have become standard issue in the Rialto Police Department, which was an early adopter of the technology and was among the first to systematically study its impact. The results have been eye-opening.

In 2011, the city of 100,000 had 24 complaints against officers, including use of force or excessive force. In 2012, the first year of Rialto's pilot program, in which 40 officers mounted cameras on their uniforms or headsets, that number dropped to three. In 2013, the city had four such complaints as the number of officers wearing cameras increased to about 70.

Rialto officers have come full circle and now like the cameras, which have exonerated officers when citizens made claims of abuse. Police watchdog groups like the cameras because they force police to be on their best behavior. And even the American Civil Liberties Union endorses the technology, seeing the accountability the cameras enable as outweighing the potential for intrusive surveillance.

“They appear to be the ideal solution that activists have wanted for decades,” says Mary Power, co-founder of the National Coalition for Police Accountability. “Police departments big and small from Los Angeles to New York are looking into them seriously. It’s a great idea.”

The push for use of video cameras to police the police gained steam after the acquittal of four white police officers who were caught on camera beating African-American motorist Rodney King in 1991 – sparking the costliest riots in US history. Now, a quarter-century later, a grand jury in Ferguson is poised to decide whether to indict Officer Darren Wilson on charges of manslaughter or murder in the death of Mr. Brown. Protesters say Brown was unfairly gunned down, Mr. Wilson's defenders say he acted in self-defense.

For their part, officers in Rialto can point to instances where their bodycams helped sort through a tangle of accusations. In one shooting incident, a witness’s video didn’t show a man holding a gun, but the officer’s video did. And an excessive-force complaint by a teenager who said he was hit in the face by an officer was dropped after the video showed the boy jumping on the officer’s back.

“It really helped to have the video because it proved the veracity of the officer’s side over that of his accuser,” says Capt. Randy Deanda, Rialto police spokesman. “This is happening all the time now and is eliminating the problem of ‘he said, she said.’ This is really changing the nature of policing here.”

Rialto’s experience with the cameras has been dramatic enough to have cities around the US take notice. After the Brown shooting on Aug. 9, police in Ferguson began wearing the cameras, and in September, the New York Police Department instituted a pilot program, installing 60 of the cameras in one high-crime precinct. In all, about 1,000 of the 18,000 police departments in the US are using wearable cameras, according to an MSNBC report.

Costing about $900 each, the cameras are about the same size and shape of a kazoo, which clips onto the policeman’s shoulder or eyeglasses. The camera has a 20-or-so inch cord that connects to a cigarette-pack-sized battery and storage unit that clips to a belt or fits into a pocket. In the center is a quarter-shaped, “on/off” button.

A new report by the Police Executive Research Forum, analyzing responses from 254 police departments – 64 of them agencies that are using the cameras – concludes “the perceived benefits that body-worn cameras offer … largely outweigh the potential drawbacks.”

The report quotes Chief Supt. Stephen Cullen of the New South Wales (Australia) Police Force, who said, “After testing out body-worn cameras, we were convinced that it was the way of the future for policing.”

Independent observers agree.

“These statistics seem to make it very clear that the existence of the camera has a sobering effect on both the officers and those whom the officers confront,” says Jim Cohen, an associate professor at Fordham University School of Law in New York. “It seems that when we know there is camera focused on us, we behave better.”

While the ACLU has come out in favor of the devices, it remains cautious, based on privacy concerns. The group has warned against the cameras' expansion beyond police departments and has written about what it sees as potential drawbacks.

“Are domestic violence victims hesitating to call the police for help by the prospect of having a camera-wearing police officer in their home, or are they otherwise affected?” asks Jay Stanley, ACLU senior policy analyst. “Are privacy abuses of the technology happening, and if so what kind and how often?”

Other observers say that how the cameras are used matters as much as whether they are used.

“Cameras can prevent bad behavior by police or criminals, and we should think hard about deploying them in appropriate cases. Certainly a judicious use of cameras might be a better use of tax dollars than tanks or sniper rifles or scary black uniforms,” says Neil Richards, a professor at Washington University School of Law in St. Louis. But, he says, “How we deploy technologies matters at least as much as whether we deploy them.”

He posits a long list of questions: “If we put cameras on police cars or officers, can the police pause the video? Can they edit it? Can they take the cameras off? How long would such video records be stored and under what conditions of public access?”

He notes that if public access is limited, then the police could selectively release video to advance their version of disputed facts. If public access is easy, there is the risk of exposing the private lives of ordinary people who have done nothing wrong.

In Rialto, some residents say they are still wary of the technology.

“I’m uncomfortable with police around who use them,” says Fernando Lopez, a clerk at a BoostMobile outlet on Riverside Drive.

But an interview with 22-year Rialto veteran Sgt. Nick Borchard seems to indicate that the police here really like them.

“To be honest, my group of senior officers here really didn’t want to use them at the beginning – we thought they would be problematic. But once we saw how much they help, we really want to use them all the time.”

He says he finds very little downside, except that “having an extra piece of equipment on my body with wire leading into my shirt makes me feel more like a robo-cop.”