The bodies of the five young men and one woman were discovered on Sunday by the owner of the garden, whose own son and daughter were among the victims.

"The six adolescents aged between 18 and 19 died from carbon monoxide poisoning," said a statement from police in the southern state of Bavaria.

"The cause of the leakage of the toxic gas remains under investigation."

A police spokesman said there was no indication it was deliberate.

He said a wood-burning stove was apparently used to heat the small brick outhouse, but that investigators were looking at other appliances as well.

The incident in the small town of Arnstein has plunged the local community into mourning.

Fire brigade commander Juergen Illek, who was among the rescue workers, told the Mainpost regional newspaper it had been the most harrowing experience of his career.

"I have never seen anything like it," he said.