A train derailment in a wooded area of southwestern Germany that killed three people on Sunday may have been caused by an overflowing sewer, local police and prosecutors said.

"It is believed that heavy rain in the area of the accident caused a sewage shaft to overflow," Ulm police and Ravensburg prosecutors said in a joint statement.

"The water triggered a landslide on the embankment next to the tracks, which in turn caused the derailment."

About 100 passengers were on the train when the accident occurred at around 6:10pm local time near the town of Riedlingen in Baden-Wuerttemberg state.

Ongoing investigation

Severe storms swept through the region at the time of the accident, according to weather services.

Three people died in the accident, police and prosecutors said, including the train's driver and a member of staff.

At least 41 people were injured, some of them severely.

Traffic is still suspended on the affected railway line and cleanup work will begin on Tuesday. 

The investigation is still ongoing and there is no indication of any foul play or interference with the line, authorities said.