Officer Johnna Watson, a spokeswoman for the Oakland police, said that the overall effect of the cameras has been positive and that the incident in 2011 was a learning experience.

"We have seen a reduction in internal affairs complaints since the department has been using the cameras," she said. The cameras "also assisted in investigations where an allegation has been made."

In Pittsburgh, police spent more than $100,000 on a system in 2012, only to find that the program was in violation of a Pennsylvania wiretapping law. That state is now debating a change to the law, though the bulk of other states — including Maryland — allow the cameras under most conditions.

In Maryland, the ACLU says it supports the use of cameras as long as officers record all interactions with the public, maintain the footage for an extended period of time, and allow subjects of tapes to access copies.

"It's a potentially important tool in police accountability," said David Rocah, a senior staff attorney at the ACLU of Maryland. "In most police encounters, there are only two witnesses. Video recordings can provide a significant, independent record."

But Rocah said police must limit the use of stored footage "so that it doesn't become another surveillance tool."

A study of the cameras last year reported a 50 percent drop in police use-of-force incidents in Rialto, Calif., a city of about 100,000. The study also reported a nearly 90 percent drop in citizens' complaints over the course of a year.

Steve Tuttle, a spokesman with Taser, the Scottsdale, Ariz.-based company that makes and operates camera systems for more than 800 agencies, said the recordings are simply an extension of recording already taking place via in-car cameras and civilians' cellphones.

Officers should be recording, Tuttle said, "because if you don't do it, somebody else is with their flip phone. They don't capture why the officer did it. It's not going to be from that officer's perspective."

The company does not have access to the footage recorded by police, Tuttle said.

He referred to a video of police hitting a man with a baton in Mesa, Ariz. A bystander's video only captured the officers hitting the man, but didn't capture what caused their reaction. The department released video from an officer's camera that showed the man struck one of the officers first.

In Laurel, a Prince George's County community of about 25,000, Officer Waddell can activate his camera with the click of a button on his belt. He does it each time he climbs out of his cruiser, whether to speak with a group of high-schoolers about a vandalism case or when he pulls over a Honda Accord for improper window tinting.

His camera, which looks like a pen, is on at all times. When the sun goes down, he stows his glasses and attaches the device to a slender headband.

The camera is always on, but does not save anything until Waddell pushes the button. He activates the device when he enters a private residence but must ask for permission to continue recording.

At the end of his shift, he dumps the camera at the roll call room in headquarters and the data is uploaded to cloud storage maintained by Evidence.com, a service offered through Taser.

Waddell can review the footage he captures throughout the day, either at a computer at headquarters or through a smartphone application. But he can't delete or edit it.

The officer can flag a video if he thinks it might be needed as evidence or if he feels a complaint might arise from the incident. All video is stored for at least 181 days, and longer if it's flagged.

He can only review footage from the camera issued to him, but his supervisor can look at his videos and those of other officers.

Waddell recalled a stop in which he ticketed a driver for expired tags. Waddell said the driver, a black man, filed a complaint saying the officer treated him differently because of his race. Waddell, who is white, said the driver was not obeying instructions to stay in his car. The department was not using cameras at the time.

The officer was cleared of wrongdoing, and the complaint was dismissed. But he said if he had been wearing a camera, the footage could've been reviewed on the spot.