Like millions of motorists, Eric Hanson used a Global Positioning System device in his Chevrolet TrailBlazer to find his way around. He probably did not expect that prosecutors would use it, too  to help convict him of killing four family members.

Prosecutors in suburban Chicago analyzed data from the Garmin G.P.S. device to pinpoint where Mr. Hanson had been on the morning after his parents were fatally shot and his sister and brother-in-law bludgeoned to death in 2005. He was convicted of the killings this year and sentenced to death.

Mr. Hanson’s trial was among recent criminal cases in which the authorities used such navigation devices to help establish a defendant’s whereabouts. Experts say such evidence will almost certainly become more common in court as the systems become more affordable and show up in more vehicles.

“There’s no real doubt,” said Alan Brill, a computer forensics expert in Minnesota who has worked with the Federal Bureau of Investigation and the Secret Service. “This follows every other technology that turns out to have information of forensic value. I think what we’re seeing is evolutionary.”