PERUGIA, Italy — As tomb heists go, it was an odd job.

The robbers were not professional tombaroli, the looters of ancient sites who have over the centuries despoiled countless graves in Italy. They were people, the authorities said, who had stumbled onto a trove of important Etruscan artifacts a decade ago while digging to build a garage in a villa just outside the city center here.

Rather than notify authorities, investigators say the looters divided up the stash and looked around for years before trying to cash in on their good fortune.

But two years ago, when the police were searching a home in Rome, they turned up a photograph of what appeared to be an illicit artifact. That investigation eventually led them to Perugia, and when the looters appeared ready to sell the artifacts this year, members of the police art theft squad moved quickly.

“We didn’t want to risk losing track of them” because “very important pieces like this would likely have ended up abroad because they are difficult to sell in Italy,” said Maj. Antonio Coppola, one of the investigators.