A tree, planted in memory of the first victim of the German far-right terrorist group, NSU, has been destroyed by suspected neo-Nazis, authorities said Friday.

German Chancellor Angela Merkel's spokesman Steffen Seibert strongly condemned the attack and called for a full investigation into the crime.

"The killing spree of the NSU, which has been undetected over a long period of time, really is a shame for Germany," Seibert said during a regular press conference in Berlin. "Remembering victims of this killing spree ... We owe this to the victims, their families but also to ourselves ... to our struggle for democracy and pluralism in our country," he stressed. The memorial tree for Enver Şimşek, the NSU's first victim, was planted last month in the eastern town of Zwickau, where the terrorist group's three members lived from 2000-2011.

The shadowy neo-Nazi group killed at least eight Turkish immigrants, a Greek worker and a German policewoman in seven years from 2000-2007. The German public first learned about the group's existence in 2011 when two of its members died during a bank robbery attempt.

Until 2011, Germany's police and intelligence services ruled out any racial motive for the murders and instead treated immigrant families as suspects in the case and even harassed them for alleged connections with mafia groups and drug traffickers.

Georg Maier, Interior Minister of Thuringia where NSU members hailed from, says the murders are still not completely clarified. Speaking to Die Welt Friday on the murder of police officers Michele Kiesewetter by the gang, Maier said there were "still many questions unanswered."

"There are so many things about this case that I can no longer believe in coincidences," Maier said. He said NSU going undetected for years was "a failure of the state." He also linked the worrying rise of far-right extremism to "this attitude of indifference" to organizations like NSU.