The fire broke out at 7:30 am on the city's 719 line. The 44-year-old bus driver saw the flames in his rear view mirror and was able to quickly quell the fire with an on board extinguisher, police reported.
Shortly before the fire began, the driver had let a group of students off the bus.
Criminal investigators determined that the incident was likely intentional, but could have been negligent arson, because it started on a bus seat.
The cost of damages has not yet been determined.
Meanwhile, more disturbing details of the deadly Hannover bus fire were released on Thursday. Police discovered a 13-year-old girl was among the dead. She was returning from a day trip to Prikings-Hof with her parents when the bus allegedly caught fire due to someone smoking in the bus restroom.
Police said Thursday that identifying the 20 bodies could take days because they were so badly burned.
Experts from around the country are currently examining the bus for clues to the cause of the devastating blaze.
It was Germany's worst coach accident for 16 years.