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 the less than 1 percent of the state's 31,500 parolees serving part of his parole on electronic monitoring.

Experts say that the technology is sound but that it can't prevent parolees from committing more crimes.

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

Ronald Robinson, 32, of Pittsburgh, wore a monitoring anklet 24 hours a day, linking him via the phone in his house to a 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.

Confidentiality rules prevent the state parole board from disclosing information about Robinson's situation, including details about violations, other than that he had more than one, spokesman Leo Dunn said.

In Pennsylvania, parole agents and supervisors decide on a case-by-case basis who gets electronic monitoring. About 240 Pennsylvania parolees are on it on any given day.

Electronic monitoring can be used for parolees who have had technical violations or to help ensure that they abide by curfew requirements, officials said.

"Let's face it: Everyone's going to get home a little late. The agent will determine how to act depending on what he knows about the parolee. There is some discretion to it," Dunn said. If a problem persists, parole requirements may be modified.

Dealing with violators is "progressive, proportional, and corrective based on evidence," he said.

William Bales, a criminology professor at Florida State University, recently completed a two-year study for the National Institute of Justice that included research on electronic monitoring in Florida. His data show that electronic monitoring appears to reduce the chances that someone will flee or commit more crimes, he said.

The use of electronic monitoring has increased over the years, Bales said, as budget-strapped states confront the cost of housing prisoners. But he said research into its effectiveness has lagged.

In the Pittsburgh case, Robinson was paroled after serving the minimum of a 21/2-to-5-year prison sentence for illegal handgun possession. He was released on Aug. 26, 2007, and was scheduled to complete his sentence Feb. 24.

While on parole, Robinson was required to submit to urinalysis, hold a job or take job training, and avoid contact with drug sellers and users, among other things.

Robinson is charged with two counts of homicide in the shooting deaths of Crawshaw, 32, and Morton, 40. Crawshaw was first to respond to a 911 call made around 8:20 p.m. from Morton's home.

Morton was found dead in his house and Crawshaw was shot and killed in his patrol car outside the house as he waited for backup to arrive.

Robinson turned himself in and is held without bail in the Allegheny County Jail.