Police in the south German state of Bavaria have detained a suspect during a fire at Adolf Hitler's Alpine mountain retreat, the Kehlsteinhaus, also known as the Eagle's Nest.
Dispatchers in Traunstein received a fire alarm from the building at 21:15 (19:15 GMT) on Friday, and firefighters found it engulfed in flames.
Difficult terrain was hindering efforts to extinguish the blaze and it was still burning early on Saturday, police said, with some 200 emergency personnel deployed. There were no reports of injuries.
The Nazi dictator used the retreat, which lies above the town of Berchtesgaden, as a part-time seat of government and met commanders there to plan the invasion of the USSR in 1941.
A criminal investigation is under way to determine whether the suspect is connected to the fire.
Hitler also used the area as a backdrop for propaganda films portraying him as a lover of animals and nature.