Belgium ? A man stole $28 million worth of diamonds from an Antwerp bank where he had been a trusted customer for a year using a stolen Argentine passport, officials said Monday.

Prosecutors say the suspect broke into safety deposit boxes in an ABM Amro bank in the city’s diamond quarter last week. He made off with diamonds weighing 120,000 carats, police said.

Police believe he may have carefully planned the robbery, becoming one of several trusted diamond traders with access to the vault. The suspect had been a regular customer at the bank for the past year, giving the name Carlos Hector Flomenbaum from Argentina.

Authorities now believe that he was using a false identity because a passport in that name was stolen in Israel a few years ago.

Police offered a $2 million reward for tips on the man’s whereabouts. They released a composite photo of a gray-haired man, 6 feet 3 inches tall and aged between 55 and 60. They said he speaks English with an American accent and often wears a baseball cap, and they are appealing to anyone who got to know him during his time in the city to come forward.

The bank discovered the theft on March 5, believing that someone took the stones that Monday morning or the previous Friday from a vault used by pawnbrokers and diamond cutters.