BERLIN — The head of Germany's domestic spy agency admitted Thursday that his office made mistakes that allowed a small neo-Nazi group to operate under the radar on a seven-year spree in which they are suspected of killing nine immigrants and a policewoman.

Heinz Fromm, who has already submitted his resignation over the case, told a special parliamentary commission there were serious shortcomings in the investigation.

"This is a serious defeat for the German security services," said Fromm, who steps down at the end of this month.

The National Socialist Underground is suspected of killing eight Turkish men and a Greek between 2000 and 2006 and a policewoman in 2007 in attacks across the country. For years, authorities suspected organized crime rather than racist violence.

Fromm said information was not shared well enough between the state and federal offices of his agency, the Federal Office for the Protection of the Constitution.

He also said the group's activities did not fit traditional patterns of right-wing violence, and his office failed to see their attacks for what they were.

"Ten executions of unsuspecting and defenseless people over a period of seven years – that is unprecedented," Fromm said, adding that he had decided to ask for early retirement to make way for a fresh view from the top. His successor has not yet been chosen.

In the end, it was not top-level intelligence but simple police work that uncovered the group's existence. After a failed bank robbery in the central city of Eisenach in November, police tracked the group's suspected founders, Uwe Boehnhardt and Uwe Mundlos, to a mobile home, which was on fire when the authorities arrived. Both were found dead there in an apparent murder-suicide.

A third alleged core member, Beate Zschaepe, turned herself in and remains in custody pending trial.

Police found the slain policewoman's service weapon in the mobile home, then discovered a pistol used in the other victims' killings at a burned-out apartment used by the group and allegedly torched by Zschaepe.

Investigators also found copies of a propaganda video featuring pictures of the victims and a cartoon image of the Pink Panther standing next to a placard proclaiming "Germany Tour, 9th Turk Shot."

Zschaepe is accused of founding and being a member of a terrorist organization. She has kept silent on the case while prosecutors prepare formal charges.

Last week, revelations that an official with the federal agency destroyed intelligence files shortly after the lid blew last November on the neo-Nazi group added to authorities' embarrassment about the case.

Fromm said he had talked with those responsible but had not been able to determine why the decision was taken to destroy the files.

"I have no convincing explanation to give," he told the parliamentary committee, which has been investigating the case.

Ahead of Fromm, the committee questioned behind closed doors the official responsible for destroying the seven files.

The files documented efforts to recruit informants in another far-right organization to which the alleged NSU members once belonged. Regardless of whether the files contained any useful information, their destruction prompted questions as to whether someone was attempting a cover-up.

The official, identified only as "M," invoked his right to refuse to testify about why he destroyed the files, lawmakers said – though he was willing to talk generally about the agency's file-keeping practices.

The official was not involved with the operation the files referred to "and we have received no other indications that anything was deliberately supposed to be covered up," said Eva Hoegl, a panel member from the opposition Social Democrats. But she said that suspicion "could not be dispelled" since the official didn't address his motives.

The committee already has determined that no members of the National Socialist Underground were among the agency's informants.

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Geir Moulson contributed to this story.