The bank has ran its own internal investigation and found no evidence of wrongdoing but prosecutors are still going to take a look-see themselves. The investigation, being conducted by economic-crimes prosecutors in Mannheim, was started in March against unnamed employees after a tax inquiry in the southwestern state of Baden Wuerttemberg identified suspicious transfers of funds from Germany to Switzerland, allegedly executed by a German taxpayer with the assistance of the Frankfurt-based office of UBS Deutschland AG...In May, prosecutors seized more than 100,000 computer files and other records during a search of the bank's Frankfurt office, Mr. Lintz said. Tax investigators are assessing this material to identify evidence that bank officials acted as accessories to tax evasion, he said. In its statement, UBS Deutschland said that "an internal investigation into the specific allegations has not identified any evidence of misbehavior by UBS Deutschland AG." It said it is cooperating with the criminal investigation. Several investigations of UBS clients in Germany are under way by local prosecutors independent of the Mannheim investigation, Mr. Lintz said. He declined to say how many clients are under investigation. A report in the Thursday edition of German newspaper Stuttgarter Nachrichten said thousands of bank clients are under investigation. Mr. Lintz declined to confirm that figure. Germany Probes UBS Staff on Tax-Evasion Allegations [WSJ]

In other words, the Swiss bank is being investigated for being a Swiss bank.