Germany on Thursday held a memorial ceremony and observed a one-minute silence for 10 murder victims of a neo-Nazi killing spree, with Chancellor Angela Merkel apologizing to their relatives for devious investigations that failed to uncover what's behind the crimes in past ten years.

"The murders of the Thuringian terror cell were an attack on our country... They have brought shame upon our country," Merkel said in the ceremony held in central Berlin's Concert House, attended by relatives of the victims and high-level officials and parliament members.

Ten candles flickered in the central stage, representing the victims, including nine immigrants -- eight Turks and one Greek --mostly businessmen and shop owners, as well as one German policewoman.

They are believed to be killed in a series of murders by a far-right, neo-Naiz terror group called the National Socialist Underground (NSU) between 2000 and 2007.

However, the murderers remained untraced for years until last November, when two suspected founders of the group were found dead in a caravan following a bank robbery, and another core member, Beate Zschaepe, exploded her rented apartment and gave herself up to police in the city of Zwickau in the state of Saxony.

Police then found the killed policewoman's weapon, a pistol used in murders and a film showing the victims' bodies in the ensuing search of suspects' living places.

Merkel said she has "never yet seen anything more inhuman" in her life while commenting on the video.

The neo-Nazi group's activities shocked entire Germany and raised questions that why the authorities have failed to find xenophobic motives behind these killings and instead looked forclues in the mafia and drug-dealing fights, and even suspected victims' relatives of crimes.

"The background to these murders lay hidden for far too long. That is the bitter truth," Merkel said. "For years, some relatives themselves unfairly faced suspicion-- that is particularly oppressive."

"For that, I beg your forgiveness," the chancellor told relatives of victims.

Semiya Simsek, whose father was shot at his flower shop in 2000, said her family had been loaded with heavy pressure for years asthe authorities suspected that some family members, even her mother, could be the killer.

"For 11 years, we couldn't even be victims with a clean conscience," she said. "My father was murdered by neo-Nazis. Is that supposed to console me?"

The German government has offered each of the victim's families at least 10,000 euros (13,240 U.S. dollars) for compensation.

Until now, five people, suspected of assisting the NSU for weapons and ammunition, have been arrested. A parliamentary inquiry committee was established to investigate what was wrong with the intelligence agencies and security authorities in the case of the killing spree.