Hanover, Germany - German investigators sought Wednesday to find out how a fire began in the toilet of a coach and quickly spread through the vehicle, killing 20 pensioners on a day's outing.

Uwe Schuenemann, interior minister of Lower Saxony state, stressed the cause of the Tuesday evening inferno, on a highway just outside the northern city of Hanover, had not been discovered yet.

Prosecutors opened a criminal inquiry to establish if any person had caused the fire, which gutted the passenger space but left the lower half of the coach largely untouched. The bus was towed to a police compound during the night with the remains still inside.

Passengers said they saw smoke come from the closed toilet, and when the door opened, flames roared out and spread through the vehicle's interior as the bus was moving along an autobahn.

There was speculation a passenger may have surreptitiously smoked in the toilet. In Germany it is illegal to smoke on public transport.

The following morning, accident investigators were studying the wreckage and pathologists began identifying the charred bodies from Germany's worst bus disaster in 15 years.

Identification was hampered because the bus manifest listed more persons than were actually on board, after some missed the trip.

Thomas Doepper, chief executive of the excursion destination Prickingshof, said all the passengers had been regular guests.

Prickingshof sells tickets to the elderly for a day in the country with coffee, a snack and the opportunity to try and buy electrical goods, bedding and other products.

Many of the passengers were lame. Rescuers recovered Zimmer frames, used by elderly people as wheeled supports to walk, from the wreckage.

Uwe Binias, Hanover chief of police, said, "The flames spread like wildfire." The driver immediately pulled over to the hard shoulder and opened the doors. He and 12 passengers aged 45 to 80 near the door escaped.

Germany's worst bus disaster in recent decades was in September 1992 when 21 were killed in a Black Forest crash. (dpa)