A bus fire that killed 20 people in northern Germany may have been caused by a spark from the undercarriage, prosecutors said Friday, discounting an initial theory that the blaze was sparked by a cigarette.

Testimony from the 13 survivors of Tuesday night’s fire established that it started in a bathroom in the middle of the bus, said local fire chief Bernd Keitel.

Passengers told officials they saw smoke seeping from behind the bathroom door. When they opened it, flames quickly engulfed the bus, trapping those seated to the rear.

But Keitel said it was unlikely that the blaze was sparked by a passenger smoking clandestinely _ smoking is illegal on buses in Germany _ in the bathroom.

Keitel told reporters he instead suspected that a spark in the bus’s undercarriage may have set light to flammable gases. It was unclear what the origin of the gases could have been.

The bus was carrying 32 passengers aged between 46 and 79 and a driver, according to police. The 51-year-old driver, who was not identified, was the only person so far to have been discharged from the hospital. Three people remained in critical condition with severe burns.

Georg Wessling, a spokesman for the Lower Saxony state’s justice ministry, said prosecutors would investigate whether someone on the bus or elsewhere might be responsible for negligent manslaughter. He said there were no suspects, and added that authorities have not yet been able to identify any of the 20 victims, due to the severity of the fire.

Autopsies and DNA testing have been ordered.

The Mercedes Benz bus had been chartered by a tour organizer to take a group of seniors on a day trip to a farm near Hannover. It was on its way back to the city when the fire broke.