The killer of Lindsay Hawker, the British teacher who was strangled and buried in a bathtub of soil in a flat near Tokyo, has almost certainly committed suicide, Japanese police have concluded.

Officially, police are still actively looking for Tatsuya Ichihashi, the 29-year-old unemployed man in whose apartment Ms Hawker’s body was found in March last year. But senior detectives believe that he would have been caught by now if he was alive, and that he must have killed himself in a remote spot where he may never be found.

The case is an embarrassment to police in Chiba prefecture, a commuter area east of Tokyo where Ichihashi lived. They let him slip through their fingers when they visited his flat as part of their search for Ms Hawker, who had been reported missing.

Despite distributing tens of thousands of wanted posters, with simulated images showing Ichihashi as he would look dressed in drag or with dyed hair, there has been no confirmed sighting of him.

“Japan has so many mountains, and if he has gone into a remote woodland and died there, they might never find him,” a police source said. “The Chiba police are continuing their investigation, but surely they will reduce it soon. The detectives in Tokyo have concluded that Ichihashi is dead.”

Ms Hawker’s father, Bill, said that Chiba police had told him on a recent visit to Japan that they had not yet reduced the 140-strong investigation team, but planned to do so.

“They’ve replaced some of the staff with fresh faces, which encouraged me because some new ideas might make them more active,” he said yesterday. “But this thing about him being dead could be a ploy to allow them to reduce the investigation while saving face.”

Ms Hawker, 22, went missing after meeting Ichihashi in a coffee shop close to his home in the suburban town of Gyotoku in Chiba. He had approached her at a railway station as she was commuting home a few days earlier, and followed her to her flat, eventually persuading her to give him a private English lesson. When friends reported her missing, police visited Ichihashi after finding his telephone number on a piece of paper in her flat.

He escaped in bare feet from eight officers. Afterwards, they found her body in the bath on the balcony.

Her hands and ankles had been tied with plastic cord used to bind plants, and she was buried in horticultural soil. A postmortem examination showed that she had been beaten all over her body, although police have refused to confirm or deny whether she was sexually assaulted.

Since then there have been many reported sightings of Ichihashi. Police have made extensive searches of areas around his flat and gay bars in Tokyo, where, according to the Japanese media, he was a regular customer.

Detective Chief Inspector Ally Wright, a senior murder investigator from the Hawkers’ home county of Warwickshire, has visited Japan with Mr Hawker and taken part in raids on 24-hour internet cafés where young homeless people often spend the night. The Hawker family also receive a weekly bulletin outlining the course of the investigation.

“The bulletin goes into too much detail - it reads like a cut and paste job,” Mr Hawker said from his home in Brandon, where he is a driving instructor. “Sightings have got us nowhere. What we need is for people who spot Ichihashi to follow him and keep following him while they call the police.”

He added: “I’ve got no idea whether he’s alive, but I hope so, and I hope he’s caught so he can atone for his crime. If he took his own life, that’s just a way out for him.

“I’m 55 and I’m not going to give up. I’ve got a pension coming at 60, and I can tell you where I’m going to be spending most of my time - over there, looking for him and making sure that others are looking for him too.”

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