For a growing number of police officers in the United States, miniature video cameras are becoming as much a part of the uniform as a pistol, handcuffs and badge.

"I've been doing this for so long, I'm just used to it," said officer Aaron Waddell of the local police force in Laurel, a Washington suburb of 25,000 that introduced body cameras nearly two years ago.

The rules are simple: whenever Waddell gets out of his patrol car for a traffic stop, or makes contact with someone on the street, he simply pushes a button on his belt.

Immediately, the pen-sized camera, attached to the frame of his glasses, starts recording everything the officer sees and hears.

"I just hit it," explained Waddell to AFP. "I don't even think about it. It's just a habit."

Interest in such devices has surged in the United States since the fatal shooting of unarmed black teenager Michael Brown by a police officer on August 9 in the Missouri town of Ferguson.

That incident -- in which a body camera was not used -- prompted daily protests, frequent nightly confrontations with riot police, and a national debate on race and law enforcement.

It also saw more than 150,000 people signing an online petition on the White House website calling for a "Mike Brown law" requiring all state, county and local police to wear body cameras.

"The law shall be made in an effort to not only detour police misconduct (i.e. brutality, profiling, abuse of power), but to ensure that all police are following procedure, and to remove all question, from normally questionable police encounters," the petition stated.

- US administration eyes idea -

President Barack Obama's administration responded positively to the idea, saying the Justice Department "continues to research best practices for implementation."

In Laurel, police chief Richard McLaughlin said: "It's all about being transparent ... and providing protection, not only to my officers but to the community we work with."

Currently, about half of the 45 officers who go out on patrol in Laurel are equipped with the body cameras, manufactured by Taser. In the coming weeks, that should go up to three-quarters.

Officers are obliged to inform the people they come into contact with that the camera is operating. They also have to ask for consent or refusal before using the device within a private home.

Each camera costs $1,600 to $2,000, including cloud storage on Taser's evidence.com website. Images are destroyed after six months unless they're needed for court purposes.

Using a password, officers can look back on a recording, but "they cannot delete it, they cannot modify it -- all they can do is view it," McLaughlin said.

- A civil rights plus? -

For the American Civil Liberties Union (ACLU), "cameras have the potential to be a win-win," said Jay Stanley, a senior policy analyst with the watchdog group's speech, privacy and technology project.

"This is technology that has real potential to benefit citizens" and curb police abuse, while protecting officers from false accusations, he told AFP.

That said, the ACLU insists that police officers cannot pick and choose who they record, nor can recordings be altered -- and as for images irrelevant to any investigation, they must be swiftly erased.

Taser, Wolfcom and Vievu are the market leaders in body cameras, having equipped hundreds of police forces around the United States.

Taser in particular sold four times more cameras in the past year than in the previous 12-month period.

Among big cities, New York (with 34,500 officers) and Washington (about 4,000 officers) have started putting body cameras to the test.

"It's very rare that we're not being videotaped somewhere by somebody anyway," said Washington police chief Cathy Lanier this week. "We're the last people to get cameras, right?"

Body cameras can act as "an independent, unbiased witness" in situations where there might be conflicting versions of what happened, said Lanier as her force deployed five different models on the street for assessment.

Last year, a study in Rialto, California, which has about 100 police officers, indicated a 87.5 percent reduction of complaints against police and a 59 percent drop in the use of force by officers.

Laurel has no precise data, but the benefits are clear.

When an officer encounters excitable, argumentative or confrontational individuals, "as soon as they realize there is a camera on, they become very cooperative, very cordial -- it totally changes the attitude."

"It forces people to be more accountable. It makes them realize, you're being recorded (and) you can't argue with video evidence."