The deaths came when a tourist bus caught fire late on Tuesday in the German city of Hanover.
Most of the passengers were elderly.
Another 13 people were injured and three of them are currently in intensive care.
"I am deeply shocked by the report of numerous deaths in a tragic bus incident near Hanover," the letter says. "I ask you to accept my deepest condolences, as well as support for the family and friends of the dead."
The fire broke out in the toilet of the bus when the vehicle was moving on a freeway, and quickly engulfed it. The driver, however, managed to stop the bus and open the doors, allowing some passengers to escape.
The fire was apparently caused by one of the passengers who had been secretly smoking in the toilet and failed to extinguish his cigarette properly. Another theory being put forward says the tragedy could have been caused by a spark in the bus's undercarriage.
All the bodies were burned beyond recognition and have yet to be identified.
The accident is the deadliest bus accident in Germany since 1992, when 21 people were killed and 35 injured in a crash in the Black Forest region of the country.
