When the door was opened, flames shot out and quickly engulfed the bus, he said.
"Passengers who were sitting close to the exit could get away, but the others had no chance," fire department official Claus Roehrbein said.
In addition to the deaths, thirteen people were injured in the blaze, including three with serious burns, according to the fire department.
The bus had 39 primarily elderly passengers and the driver aboard, according to the Hannover-based company Mommeyer that owns it. Wittke, however, said 33 people were aboard the bus. It was not possible to immediately reconcile the difference.
The bus had been chartered by a tour group to go from Hannover on a day trip to a farm, Uwe Prehn, the husband of the bus company owner, told The Associated Press by telephone.
It was on its way back to Hannover when the fire broke out at 8:45 p.m. (1945 GMT, 2:45 p.m. EST). Wittke said all the passengers were local people.
Firefighters combed the scorched wreckage of the bus early Wednesday, trying to put out the fire. The bus had pulled off into an emergency lane before it was engulfed in flames and its back window had been blown out.
Transport Minister Wolfgang Tiefensee said he was "deeply shocked" by the accident and called for a careful investigation into the cause of the fire and into whether safety regulations were followed.
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