HARRISBURG - Footage from police vehicle dashboard cameras has long been broadcast on television and posted online, and there has been much debate about the use of body cameras for police since the fatal shooting last year of 18-year-old Michael Brown in Ferguson, Mo.

But the existence of cameras attached to stun guns was a surprise to a Pennsylvania prosecutor, who said a recording made by an officer's own stun gun was the strongest evidence used to charge her this week with homicide in a man's shooting death in February.

"I did not know until this case occurred" that some stun guns include cameras, Dauphin County District Attorney Ed Marsico said this week. "Many of us in law enforcement did not know."

For Taser International Inc., which has sold about 800,000 stun guns, nearly all to police and military agencies, the cameras are a popular feature, with current models able to record in high definition and color. The company has produced about 80,000 stun guns with cameras.

Ed Primeau, a video forensics expert in Rochester Hills, Mich., said footage from cameras mounted on stun guns was increasingly finding its way into criminal and civil cases.

"When you see where that gun is pointed, it's a video accounting of the credibility as to why that police officer fired," Primeau said.

Steve Tuttle, spokesman for Taser International of Scottsdale, Ariz., said the company had been producing stun guns with cameras since 2005. A camera adds about $500 to the $1,200 cost of Taser's basic model, the X2.

The cameras become active when the device's safety is turned off.

"For the most part, they've been a tremendous tool to take out the doubt and uncertainty when a Taser device is deployed," Tuttle said, "and what the circumstances were during an incident."

In the Pennsylvania case, police on Tuesday charged Hummelstown Police Officer Lisa Mearkle with criminal homicide, alleging she shot an unarmed man, David Kassick, as he lay face down after an attempted traffic stop and vehicle pursuit.

Her lawyer, Brian Perry, said Mearkle purposely kept her Taser pointed at the man to document what occurred, not thinking it would be used against her. She shocked him four times before shooting him when he did not comply with her commands to show his hands, police said.

"Lisa was aware that the Taser contained a video camera, and she held the Taser trained on Mr. Kassick, knowing that the incident was being recorded," Perry said Wednesday. "That's one of the reasons she's so adamant she didn't do anything wrong."

In the affidavit used to charge Mearkle, investigators said they reviewed the video many times.

"At the time Officer Mearkle fires both rounds from her pistol, the video clearly depicts Kassick lying on the snow-covered lawn with his face toward the ground," they wrote. "Furthermore, at the time the rounds are fired, nothing can be seen in either of Kassick's hands, nor does he point or direct anything toward Officer Mearkle."