The Internet vandals hijacked the welcome page at www.buchenwald.de, in remembrance of victims of one of the largest and most notorious concentration camps on German soil in World War II, said foundation director Volkhard Knigge.
The site, which was partially accessible late Wednesday, was defaced with slogans such as "Brown is beautiful" - in reference to the shirts worn by Nazi stormtroopers - and "We will return," Knigge said.
The site includes archives in memory of the estimated 56,000 people from all over Europe who died at Buchenwald in the state of Thuringia between 1937 and 1945, starved and worked to death, killed in medical experiments or summarily executed.
A total of around 250,000 people were imprisoned in Buchenwald and in its 136 nearby sub-camps where prisoners carried out forced labour in factories for the Nazi war effort.
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They included Jews, Roma, Sinti, homosexuals, the disabled, Jehova's Witnesses and real and imagined political opponents of Hitler from around Nazi-occupied Europe including France, Ukraine, Poland and the Netherlands.
Police are investigating the incident.