Swiss police said this evening they were holding a man who lost his wife, son and daughter in an airliner crash on suspicion of killing the air traffic controller who had been on duty the night his family died.
Police and prosecutors declined to give the suspect's nationality but said he had first attracted suspicion by his odd behaviour at an anniversary service in July for the 71, mainly Russian, victims of the mid-air collision of two planes in 2002.
The Danish-born controller, stabbed to death on the doorstep of his family home near Zurich airport on Tuesday, had been in charge of traffic over Lake Constance when a holiday charter carrying more than 50 Russian children collided with a DHL cargo jet.
Police have said revenge might have been the motive for the attack on the 36-year-old controller, who died from multiple stab wounds in front of his wife and children.
Witnesses said the killer was a burly man who spoke broken German and who had asked directions to the controller's home.
"One of the options was that the assailant was to be found among the victims' families," Zurich prosecutor Mr Pascal Gossner told a news conference.