BRUSSELS, Belgium (AP) – 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.

Police did not say why they waited over a week before making the theft public, nor did they mention who had put up the reward money.

In 2003, in the world’s largest safe-deposit box theft, thieves in Antwerp pried open 123 boxes, finding so much loot they could only carry away $100 million worth of diamonds, gold and jewelry.

More than half the world’s diamonds are traded in Antwerp’s gem district, a maze of tiny streets hugging the main train station. Its turnover of $23 billion a year makes it one of the densest concentrations of valuables on earth.

Jewish traders have been there for five centuries. Bearded men in broad black hats still predominate, but Indian dealers have also moved in to the many trading houses and glittering gem stores.

Huge deals are sealed with a handshake and most of the diamond trade is done on trust without a trace of paper. However, that trust is reinforced with heavy security – briefcases handcuffed to wrists, cameras filming the milling crowds from many angles, a special police station, and circles of steel pillars at both ends of the district.