Berlin police spokesman Thomas Neuendorf says the tunnel led from an underground garage into the bank's safe deposit room.
Neuendorf told The Associated Press Monday that the tunnel was "very professional" and must have taken weeks or even months to complete. It was elaborately constructed and even had ceiling supports.
Police were alerted to the break-in early Monday when a security guard noticed smoke coming from the deposit room.
Neuendorf says police are still trying to determine what valuables were stolen from the deposit boxes.
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