The official says the thieves made off with about euro80,000 ($111,000) worth of jewelry, two laptop computers and Bellucci's Italian passport. The thieves broke into the apartment, in a four-story building in Paris' gritty 20th district, through the balcony on Sunday evening.
The official was speaking on condition of anonymity because he was not authorized to speak on the record about an ongoing investigation.
The Italian actress gained fame for posing nude in Vanity Fair magazine and starring in movies such as "The Passion of the Christ" and "Bram Stoker's Dracula."
Cassel has also been featured in American movies, starring as -- ironically -- a sly thief in the George Clooney vehicle "Ocean's Thirteen."