Yuri Kaloyev knew his brother was a broken man even before he disappeared a week ago. Two years after his wife, son and daughter, victims of a head-on plane collision over the Swiss-German border, had been laid to rest amid the sombre rows of a cemetery in their home town of Vladikavkaz, in southern Russia, his family's ghosts still haunted his nights. "You could find my brother, even at 2am, at the cemetery crying on their gravestones," Yuri Kaloyev said. "He suffered. He could not work. He locked himself away."

Yesterday, however, the desolate truth about his brother began to emerge. Descriptions given by the Swiss police of a man they have arrested for the savage stabbing to death on Tuesday of the air traffic controller widely blamed for the plane crash closely fit Mr Kaloyev.

He is 48. He lost his wife Svetlana, 10-year-old son Konstantin and four-year-old daughter Diana in the disaster in July 2002, when the Russian charter aircraft in which they were travelling ploughed into a cargo plane in the night sky above Germany. While the Russian foreign ministry have requested confirmation of the arrested man's identity, the only other man who lost his entire family in the crash, Vladimir Savchuck, has appeared on Russian TV, deploring the killing.

Before arresting their suspect on Wednesday, Swiss police admitted a relative of one of the victims of the crash might have been responsible. Yesterday, however, as fresh details emerged, it appeared that they were dealing with an unprecedented case - of deliberate slow-burning revenge by a grief-crazed relative who had nothing left to lose.

According to investigators, on Tuesday last week Vitali Kaloyev phoned a Swiss travel company and asked the firm to book him a hotel room close to Zurich airport. On Saturday Mr Kaloyev arrived in Zurich, entirely legally, and checked into the Welcome-Inn hotel in the suburb of Kloten.

Mr Kaloyev, however, chose it for another, darker reason: the suburban hotel is a short taxi ride away from where Peter Nielsen, the 36-year-old Danish air traffic controller widely blamed for the catastrophic plane crash, lived with his wife and three children.

According to hotel staff, in the two days before the murder Mr Kaloyev did little to attract attention. "He was very quiet," the hotel's manager, Simona Huonder, said yesterday. "We hardly saw him during the time he stayed with us. He was on his own the whole time, mostly up in his room." She added: "He didn't speak very good English. My colleague who checked him in had to give him information slowly."

At breakfast Mr Kaloyev ate alone, later flicking through brochures offering city tours. "He seemed like any other tourist," Ms Hounder said. On Tuesday afternoon, however, Mr Kaloyev left his hotel room - No 316 - and set off for Peter Nielsen's house, a half-hour's walk away. A female neighbour of Mr Nielsen spotted him. She then asked him what he wanted. He waved a piece of paper with Nielsen's name on it. The neighbour pointed to the air traffic controller's front door, but instead of knocking on it, Mr Kaloyev sat down in the front garden, near a bench.

Mr Nielsen, who had lived in Switzerland since 1995, had just returned home from a trip to Geneva. His wife had picked him up from the airport. He spotted the intruder, went outside, and asked him what he wanted. Swiss detectives say that the couple's three children went into the garden as well; the controller's wife then called them back, and was inside herself when she heard a "kind of scream".

She rushed out to discover her husband lying in a pool of blood. The victim and the killer who spoke "broken German" had had a brief conversation; what they said, however, is unknown.

Mr Nielsen's wife watched her husband's assailant run off; by the time the police arrived at 6.17pm it was too late. The controller, who suffered multiple injuries, had bled to death.

Clues for detectives were numerous. They had several good descriptions - of a burly, unshaven, dark-haired man in his late 40s or early 50s who appeared to come from eastern Europe or Russia. They had a murder weapon - a 22cm jackknife with a 14cm blade that had been thrown away near the scene. And they had a name: the chief suspect was a man who, police said, had "behaved strangely" during the first anniversary of the crash last summer in the German town of Überlingen. The man had allegedly threatened officials from Skyguide - the firm for which Mr Nielsen worked - and described him as "scum".

So far, however, the suspect has denied involvement in the killing. Yesterday Mr Kaloyev's brother said that in the months before the murder Vitali had slowly fallen apart, despite support from his sisters, and the traditional, strong family ties of Caucasus society. "His condition was terrible. Imagine what you feel when you lose both your beloved children and wife," he said. "He disappeared a week ago without telling anyone. And that is all I know."

It is a tragic end to Mr Kaloyev's seemingly endless grief at the loss of his family. A native of Vladikavkaz, near the border with Chechnya, he got a two-year contract to work as a builder and architect on a project in Barcelona. Just as his contract ended, in June 2002, he decided to prolong his stay in Spain, and asked his family to fly out and join him for a month's holiday. He was waiting for them at Barcelona airport when he learned of the crash.

Mr Kaloyev was one of the first relatives to arrive at the scene, and discovered the body of his daughter, still intact, almost two miles from where the accident happened. "Diana dreamed of coming with her mother and brother to see me," he wrote on a website commemorating the crash's 71 victims, most of whom were Russian schoolchildren.

Remorse

Mr Nielsen was the only person on duty when the disaster took place. He had wrongly instructed the Bashkirian airlines plane to descend, even though its onboard warning equipment told it to climb. The pilot followed the controller's instructions and ploughed into a DHL cargo plane that was descending in accordance with its own collision-avoiding equipment. Mr Nielsen expressed remorse at what had happened, but in a statement issued after the tragedy pointed out that he was not the only person responsible.

The apparent revenge killing, meanwhile, has shocked all those involved in the still-unresolved fight to gain justice for the crash victims. Yulia Fedotova, a lawyer representing the families, who lost her own daughter Sofia, 15, in the crash said she was "shocked" by the controller's murder. She added: "We still do not have any official confirmation that the murderer was Kaloyev. Mr Kaloyev's personal trauma, however, was clear to those around him."

Margarita, wife to his brother Yuri, told the Izvestiya newspaper: "Vitali suffered everything alone. And after two years, he was in such a state that I would not be surprised if he would behave irrationally. Anyone can put himself in his place: in a minute to lose all your family."

Mr Kaloyev's days in Vladikavkaz after the funeral appear to have slipped by, marked by little more than visits to the cemetery. According to Izvestiya, at the memorial service last year he took the head of Skyguide, Alan Rossier, aside afterwards and asked him "uncomfortable questions about who was to be blamed". Mr Kaloyev agreed to come to the Skyguide office the following day, the newspaper reported. According to the paper's sources, "Kaloyev asked several times: do you think the air controller is to blame? He also asked to meet him."

Yet his brother disputes the accounts. Yuri Kaloyev, who travelled with him to Switzerland and Germany to collect his family's bodies from the scene of the crash, reserves his own fury for the air traffic control company Skyguide.

"All this talk and speculation in the newspapers about his abnormal behaviour last year at the ceremony in Switzerland is rubbish. He was fine. What is abnormal is the behaviour of Skyguide who did not sack such an air controller and director as Alan Rossier."

The intensity of Mr Kaloyev's grief remains clear in the internet eulogy he wrote for his son.