Turkey's foreign minister said Saturday that a killing spree of mainly Turkish immigrants in Germany between 2000 and 2007 was "a racist attack that should not go unpunished," as he visited victims' families in Berlin. At a closed-door meeting with families, Ahmet Davutoglu vowed that Turkey would keep a close eye on the high-profile case that has shocked and shamed Germany, according to a foreign ministry official. On Monday, the hotly anticipated trial opened of 38-year-old Beate Zschaepe, accused of being at the heart of the murderous neo-Nazi cell that called itself the National Socialist Underground (NSU). Zschaepe denies the charge of complicity in the murders of eight ethnic Turks, a Greek immigrant and a German policewoman. The discovery in 2011 of the gang embarrassed authorities, exposing deep security flaws and raising uncomfortable questions about how the cell went undetected for so long in a country proud of owning up to its Nazi past. Davutoglu said that while not all Germans could be held responsible for the murders, questions still remained to be answered. "What upset us is that the German government long failed to realise that the racist gang came from German society and instead considered the Turks as a potential risk," the minister told reporters at the Turkish embassy in Berlin. He praised victims' families for their "dignified" and "self-confident" stand during Monday's trial despite what they have suffered over the past decade. After a meeting in the southeastern Berlin district of Kreuzberg, home to a sizeable Turkish population, Davutoglu called on the Turkish community in Germany to "stand tall", stressing that "Turks are essential and indispensable elements of this society." One of the victims' families said the murders were an "apparent sign of racism in Germany", adding that the minister had assured them that Ankara would stand by them. "The minister said Turkey stands behind us and will never allow this trial to drop from the public agenda," Kerim Simsek, whose father Enver was killed in 2000, told AFP. The minister also called on the Turkish community in Germany -- the largest outside Turkey roughly three million strong -- to protect their cultural identity but added they must make an effort to integrate into German society. Turks in Germany should not be inward-looking, nor should they become stuck "in ghettos" but instead should be "self-confident and open up to the whole of Europe," said the minister. "Everyone should know that integration and assimilation are two different things. Nobody can assimilate us under any circumstances," stressed Davutoglu.