Police in the southern German state of Bavaria have detained a 33-year-old German man 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 local time (19:15 GMT) on Friday and firefighters found it engulfed in flames.
Some 200 emergency personnel were deployed to the scene where the blaze was still burning early on Saturday, as police said difficult terrain hindered efforts to bring the fire under control.
Police said they apprehended the suspect riding a motorcycle near the scene and possible motives were still being examined, German media reported. A criminal investigation has been launched.
There is currently nothing to suggest the involvement of further suspects, police official Gerhart Holzinger told a press conference on Saturday afternoon.
Public prosecutor Rainer Vietze said a psychiatric report had concluded the 33-year-old man showed signs of possible mental illness and diminished responsibility might therefore be assumed.
Vietze said a possible political motive was also being considered, but stressed the investigation was in its early stages.
The residence, above the town of Berchtesgaden, was one of several buildings in the area used by the Nazi dictator and his entourage during World War Two.
Hitler and top Nazi officials used his nearby Berghof residence to plan the invasion of the USSR in 1941 and the whole complex was considered one of his headquarters.
He also used the area as a backdrop for propaganda films portraying him as a lover of animals and nature.
Local media reported that site had been "largely destroyed" and the public was urged to avoid the area.
Bavaria's Interior Minister Joachim Herrmann told reporters that the site would "under no circumstances remain a ruin", broadcaster BR24 reported. "It is important that we continue our work of remembrance," he added.
Completed by the Nazis in 1938, the Eagle's Nest, which sits at an elevation of around 1,800m (5,900ft), escaped destruction after their defeat in 1945.
Since then, it has become a popular tourist destination and houses a mountain inn.
It serves as "a cautionary reminder of the abysses of the inhumane Nazi dictatorship", according to its official website, external.
During WW2, Hitler and his followers were responsible for the deaths of tens of millions of people, including the mass murder of European Jews. Homosexuals, gypsies, the disabled and ethnic minorities were sent to concentration camps and put to death.
There has been a wider debate in Germany and Vienna over whether to preserve, repurpose or demolish buildings connected to Hitler.
While some say these buildings should be preserved for historical reasons, others have raised concerns that they could be used as monuments or places of pilgrimage for neo-Nazis and far-right extremists.
In June, Berlin authorities announced they would tear down one of Hitler's bunkers - described at the time as "the power centre of Nazi Germany" - to build flats and offices on the site.
Those critical of the plans said the bunker had "significant historical value" and that it should have been turned into a museum and memorial site with an exhibit about the end of the war.
And in July, after years of controversy, Austrian authorities refurbished the house in the town of Braunau am Inn in which Hitler was born and turned it into a police station.