PITTSBURGH - A felon accused of gunning down a police officer in his patrol car after allegedly killing another man over a drug dispute was one of fewer than 1 percent of the state's 31,500 parolees serving a portion of his parole on electronic monitoring.

Experts say the technology behind such devices is sound, but it can't prevent someone from committing a crime.

"Whether it is being used on the right people with the right parameters, there's always discussion," said Marc Renzema, a Kutztown University criminal justice professor who has studied electronic monitoring.

Ronald Robinson, 32, of Pittsburgh, wore the device on his ankle 24 hours a day, linking him via the phone in his house to a monitoring station in Harrisburg.

If he wasn't home when he was supposed to be, his parole officer would be alerted.

Allegheny County District Attorney Stephen Zappala said the Dec. 6 shootings of Penn Hills Officer Michael Crawshaw and Danyal Morton occurred before Robinson was required to be home for the night under terms of his parole.

In Pennsylvania, parole agents and supervisors decide on a case-by-case basis who gets electronic monitoring.

Two people may have committed the same crime, but because of an individual's circumstances, only one of them may get electronic monitoring, said Leo Dunn, a parold board spokesman.

About 240 state parolees are on electronic monitoring in Pennsylvania on any given day.

Renzema said that while the devices can't be expected to prevent crime, even delaying recidivism is a benefit to society.

"If you're going to let [a prisoner] out it's better to have it than not have it," Renzema said.

Robinson was paroled after serving the minimum of a 2 1/2 to 5-year prison sentence for illegally possessing a handgun.

The board approved his parole on May 3, 2007; he was released on Aug. 26, 2007.

He was scheduled to complete his sentence Feb. 24.

It's far less costly to have someone on parole than in prison. Dunn estimates it costs about $3,500 a year to supervise a parolee compared with about $33,000 to keep someone in prison.

Dunn said most parolees don't get into trouble. A recent state study found that 95 percent of parolees did not get convicted of a new offense in 2008.

The parole board has said it has looked into the handling of Robinson's case while on parole and found nothing amiss.