ZURICH--Swiss prosecutors Thursday said they have started an investigation into the theft of bank data at Swiss bank Julius Baer Gruppe AG (BAER.VX) that was allegedly bought by German authorities trying to crack down on suspected German tax dogders.

The office of the General Attorney also said a suspect has been arrested in connection with the case, but declined to provide additional information.

Julius Baer said it was not in a position to comment on ongoing legal proceedings. Chief Executive Boris Collardi confirmed in a recent interview with the Swiss daily SonntagsZeitung that the bank had fallen victim to data theft. A spokesman at the bank confirmed Mr. Collardi's comments.

Local reports say the compact disc containing data of Julius Baer clients was sold by a Zurich-based employee of the bank to German authorities.

Germany and Switzerland became embroiled in a major row in 2010 when German authorities raided branches of Credit Suisse Group AG (CS) in 13 German cities after buying data on suspected tax dodgers.

Switzerland reacted angrily, saying the data--bought for a reported 2.5 million euros ($3.1 million)--were stolen in violation of its banking secrecy laws.

A tax treaty between the two countries still has to be ratified by the upper house of the German parliament, where majority opposition social democrats and Greens consider it to be too soft on evaders.

Write to Marta Falconi at marta.falconi@dowjones.com

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