IT had been 623 days since the two planes collided over Lake Constance when the man with the knife arrived at Peter Nielsen’s home in Zurich. Police believe he had come for revenge, blaming Mr Nielsen, an air traffic controller, for the crash that killed 71 people, 52 of them Russian children heading for a beach holiday in Spain. The two men had a brief conversation, the assailant speaking in broken German before he stabbed Mr Nielsen to death in front of his wife and two of his three children. The attacker, who was dressed in black, ran from the scene and police have yet to trace him or the murder weapon. Mr Nielsen, 36, was in charge in the control tower at Zurich Airport on the night of 1 July, 2002, when two planes collided over Lake Constance on the Swiss-German border. Full Story