The neo-Nazi killing spree by the NSU is "a very dark stain" in Germany's history, Chancellor Angela Merkel said on Friday, and promised to continue efforts to uncover facts about these murders.
Merkel made the remarks during her annual press conference, amid heavy criticism from families of far-right terror victims.
"How could such horrible, such complicated acts happen and not timely detected by the relevant authorities, and could not be resolved for a long time," she said.
"Without doubt, this is a very dark stain in the history of Germany. One cannot simply close this chapter," she added.
The shadowy National Socialist Underground (NSU) killed 8 Turkish immigrants, a Greek citizen and a German police officer, between 2000 and 2007, but the murders had long remained unresolved.
After a five-year-long trial, a Munich court last week handed a life sentence to the main suspect Beate Zschaepe and gave lighter sentences to four other neo-Nazis, but families of the victims expressed disappointment with the verdict and said the trial has left many key questions unanswered.
The families also heavily criticized Chancellor Merkel for not honoring her promise she made four years ago to uncover all the facts about NSU murders.
The German public first learned of the NSU's existence and its role in the murders in 2011, when two members -- Uwe Mundlos and Uwe Bohnhardt -- died after an unsuccessful bank robbery and police found guns and extreme-right literature in their apartment.
During the five-year trial, Beate Zschaepe, the only surviving member of the group, declined to give any insight about the NSU and tried to lay the blame on her two deceased colleagues.
Until 2011, Germany's police and intelligence services ruled out any racial motive for the murders and instead treated immigrant families as suspects, questioning them over alleged connections with mafia groups and drug traffickers.
While recent revelations have shown that Germany's domestic intelligence agency had dozens of informants who had contacts with the NSU suspects, officials insisted that they had no prior information about the NSU terror cell.
However, authorities in the central German state of Hesse have recently decided to keep several documents secret for 120 years, prompting further speculations about the NSU's possible ties.
