Swiss police have launched a hunt for the killer of an air traffic controller who had been on duty at the time of a deadly mid-air collision in 2002.
Most of those killed in the air crash were Russian schoolchildren
They are looking for a man, speaking broken German, who fled on foot after stabbing his victim in front of his wife at their home in Zurich.
Police are not ruling out a link between the killing and the crash.
Seventy-one people - mostly Russian children from wealthy families- died in the disaster over southern Germany.
The controller - a 36-year-old Danish national - has never been publicly named.
He was placed on medical leave after the crash, but had since been allowed to return to work on other duties, according to reports.
Swiss police said a link to the crash was a "central issue" in their hunt for the attacker, who is described as being powerfully built and aged around 50.
"(Revenge) cannot be ruled out," public prosecutor Pascal Gossner told Reuters news agency. "We are looking into whether there is a link between the killing and the air accident."
But added: "We will also be looking into the personal circumstances of the victim".
The dead man's employer, the Swiss air traffic agency Skyguide, said it was appalled by the killing and that its employees were "in shock".
A quarter of Skyguide's Zurich staff reportedly did not arrive for work on Wednesday.
The firm said air traffic was scaled back by 40% temporarily because of the impact of the killing.
Employees and the family of the controller were said to be receiving special counselling.
Zurich police were helping organise special protection for employees - particularly for a second controller on duty the night of the crash, Skyguide said.
The prosecutor said it was "speculation" that the murder was an act of vengeance on behalf of the bereaved families.
The man was stabbed to death in front of his wife at home
"You have to say he spoke broken German. But you cannot say he was from Russia," the prosecutor said.
A lawyer for families of the crash victims said they were distressed at such a connection being made.
"We reject any violent act... The families do not want to be associated with this," Gerrit Wilmans told Reuters.
The murdered controller was in charge of traffic over Lake Constance late on 1 July 2002 when a holiday charter carrying mostly Russian children collided with a DHL cargo jet above the town of Ueberlingen.
Skyguide has been criticised for its role in tragedy after investigators revealed that only one controller was on duty when the collision happened. His partner had been on a break.
Accident investigators said the controller told the pilot of the Russian plane to descend when its onboard collision warning equipment was telling it to climb.
Investigators also discovered that the agency's collision alert system was out of action for maintenance, and work on its telephone system meant a warning call from German colleagues never got through.
The victims included 52 Russian schoolchildren, most of them sons and daughters of the wealthy elite of the republic of Bashkortostan in the southern Urals region.
In a statement released after the accident, the air traffic controller acknowledged that errors in the traffic control network contributed to the disaster.