Authorities have arrested an Israeli citizen in Germany on suspicion that he was involved in the theft of $28 million worth of diamonds from an Antwerp bank last month, officials said Monday. Eliser Mishali was arrested in Frankfurt last week as he sought to leave the country, Antwerp prosecutor's office spokeswoman Dominique Reyniers said. Authorities were still looking for the main suspect and the diamonds. Belgian officials said they would seek Mishali's extradition. Last month, a man broke into open safety deposit boxes in an ABM Amro bank in Antwerp's diamond quarter and stole diamonds weighing 120,000 karats. The main suspect had been a regular customer at the bank for the past year, giving a name of an Argentinian national, authorities said. They now believe that the suspect was using a false identity because a passport in that name was stolen in Israel a few years ago. The bank discovered the theft March 5, believing that someone took the stones from a vault used by pawnbrokers and diamond cutters. The accomplice arrested last week had been traveling for thousands of kilometers through Germany and the Netherlands in the second half of March and early April before he was arrested. It is believed the diamonds will be difficult to sell on world markets since many of the gems cannot be sold without certificates of origin to show they have not come illegally from war zones. 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.