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

Four of the 13 survivors were in critical condition.

Hans-Oliver Rennekampff, a professor at Hanover teaching hospital in Hanover, northern Germany, said they were at risk because of burns, their advanced age and inhalation of toxic fumes from burning bus upholstery.

Police were struggling to discover the names of those killed, since there was no passenger list.

There was speculation a passenger may have surreptitiously smoked in the bus toilet and left the cigarette behind still burning. In Germany it is illegal to smoke on public transport.

But city fire brigades spokesman Bernd Keitel said, "we simply do not know the cause."

Passengers said they saw smoke come from the closed toilet cubicle, and when its door opened, flames roared out and spread through the vehicle's interior.

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.

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 46 to 80 near the door escaped.

Many of the passengers were lame.

Thomas Doepper, chief executive of the excursion destination Prickingshof, which had chartered the four-year-old bus, 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.

It was Germany's worst bus disaster in 15 years and revived memories of a September 1992 crash in which 21 passengers were killed in the Black Forest after a tired driver lost control. (dpa)