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....