A lone air traffic controller missed a key warning on his radar screen in one of a chain of errors that led to a 2002 mid-air crash over Germany which killed dozens of Russian children, a report showed on Wednesday. The report came nearly three months after the controller on duty at the time of the disaster, identified by Swiss media as Peter Nielsen, was stabbed to death outside his house.

Police are holding a bereaved Russian who lost his wife, son and daughter in the crash, in which 71 people were killed in one of Europe's worst peacetime air accidents. Swiss air traffic control firm Skyguide, which operates the airspace over Southern Germany, expressed dismay at weaknesses in its systems partially blamed for the crash.

The firm said it accepted full responsibility for its errors, detailed in a German report, that helped cause a DHL cargo jet and a Russian charter carrying 52 children heading for a holiday in Spain to collide above Ueberlingen on July 1, 2002.

"We ask for forgiveness," Skyguide Chief Executive Alain Rossier told a news conference after the report was published. "We have learned our lessons from this tragedy. ...Something like this can never ever happen again."

Skyguide was commenting on a report by German investigators which – while not blaming the air traffic controller alone – said he had issued his first instructions less than a minute before the fatal crash.

The rule that at least two controllers should be on duty had been relaxed during quiet periods at night, and just before the crash the controller's colleague had been taking a break, leaving the other alone to man two desks, the report showed.

The German Federal Bureau of Aircraft Accidents Investigation said safety procedures had not been followed before the collision near the Swiss-German border between the Bashkirian Airlines Tupolev 154 and the DHL Boeing 757.

The head of the investigation Joerg Schoeneberg detailed the errors as the clock ticked down to the crash at 23:35:32 local time. Skyguide said it would cooperate with Swiss prosecutors in their criminal probes, both against unknown people and against the dead air traffic controller on duty as well as Skyguide.

Swiss President Joseph Deiss apologised to Russian President Vladimir Putin for the "immense suffering" caused by the accident, the website of the Swiss embassy in Moscow said. Switzerland was "committed to providing swift...generous compensation to the surviving relatives," he said in a letter.

Skyguide said it had compensated 13 families and was hoping to settle outstanding claims soon. The two aircraft crashed despite repeated warnings from onboard alert systems designed to prevent mid-air collisions after the pilot of the Russian plane followed ground control instructions to descend into the path of the Boeing.

The pilot of the Boeing had already obeyed an instruction from his own onboard unit to descend. The system should have ensured the Tupolev climbed to avoid a crash. Instead, ground control ordered it twice to descend, and the pilot only began an attempt to climb seconds before the two planes crashed at a height of 34,890 feet.