A Russian engineer who lost his wife and two children in a mid-air collision confessed today to the fatal stabbing of the air traffic controller who had been in charge on the night of the disaster.

Vitaly Kaloyev's appearance in a Zurich court was the final act of a tragedy that began three years ago on a clear night over Lake Constance, when a Bashkirian Airlines Tupolev 154 crashed into a Boeing cargo plane operated by the DHL courier service.

The air traffic controller, Peter Nielsen, had been alone in the Zurich control tower and wrongly told the pilot of the Russian plane to lose height - while the DHL plane made exactly the same manoeuvre.

Some 45 children, on their way to holidays in the Costa Brava, died in the crash, including Mr Kaloyev's ten-year-old son, Konstantin, and four-year-old daughter Diana, whose body was recovered virtually unscathed since her fall had been broken by trees. Mr Kaloyev said: "She was carried to earth by an angel."

Konstantin and Svetlana hit the ground hard after falling 36,000 feet. The boy smashed on the asphalt in front of a bus shelter in the small German township of Ueberlingen. Mrs Kaloyev was found in a cornfield.

The engineer who was waiting for his family in Barcelona - he had a contract to build a villa in Spain - took the first available flight and was the first relative to see the crash site on the banks of Lake Constance. Since then he has been inconsolable.

Eighteen months after the crash Mr Kaloyev tracked down the air traffic controller, a Dane working for the company Skyguid.

On the night of the crash Mr Nielsen was working alone, with the second controller asleep. He had to simultaneously change the flight path of the two planes - with barely 44 seconds available - and land another charter plane coming in from Crete.

One of the radar emergency lights was not functioning and the main telephone was not ringing. Mr Nielsen ordered the Russian plane to lose height but the Boeing was making exactly the same manoeuvre.

Mr Kaloyev found the house of Mr Nielsen with the help of a Moscow private detective agency. He went onto the traffic controller’s balcony and called out his name. Mr Nielsen went out and closed the door behind him. There was a short argument and then Mr Kaloyev repeatedly stabbed him with a Ranger knife.

The nub of the defence case presented yesterday by his lawyer Markus Hug was that a man so distraught could not possibly be judged by normal judicial standards. The state prosecutor wants Mr Kaloyev, 48, to be jailed for "killing with intent" - which though less than murder nonetheless carries a maximum sentence of 20 years.

But Herr Hug argues that Mr Kaloyev was mentally deranged at the time of the killing and was therefore guilty only of manslaughter with a maximum sentence of 10 years. Mr Kaloyev says he remembers nothing of the killing but accepts that he did it.

"It was a crime of passion, of impulse," says Herr Hug.

Prosecutors in Germany and Switzerland are still conducting investigations against members of the Skyguid staff to establish whether they could be charged for negligent killing. For the Russian families - not just Mr Kaloyev - this investigation has been proceeding too slowly. When Mr Kaloyev attended a memorial service in Ueberlingen in 2003 he exploded with anger, complaining that Skyguid was accepting only partial responsibility for the crash.

According to the prosecutor in yesterday’s trial, that was the moment when Mr Kaloyev formed an intention to kill the air traffic controller.

The engineer has been in prison and psychiatric clinics since the stabbing in 2004. Psychiatrists say that he is fit to stand trial but various expert testimonials to be presented to the court paint a picture of a man still crippled by grief and barely able to make sense of the world. His home in northern Ossetia has become a shrine: all the beds are scattered with dolls, toys and photographs and surrounded by candles.

The engineer's life savings have gone towards building a large gravestone and tomb for the family as well as a plaque at the spot in the cornfield where his wife fell to earth.

The trial has sparked great interest in Russia and, in particular, Mr Kaloyev’s home province of North Ossetia. A prominent delegation from the province led by its president, Taimuraz Mansurov, attended the opening of today's trial.

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