The family of Lucie Blackman, the British hostess whose dismembered corpse was found buried in concrete in a cave outside Tokyo, has reacted furiously after the businessman accused of killing her was found not guilty yesterday. But they denied their decision to accept "condolence money" had influenced the judge.

Joji Obara was acquitted of all charges relating to her death by a Tokyo court yesterday, but jailed for life for raping nine other women. The victims included an Australian model, Carita Ridgeway, 21, who died after ingesting chloroform, the date-rape drug Obara favoured.

Ms Blackman's father Tim, who sat in court feet from Obara as the verdict was being read out, called it a "bitter disappointment".

"Today we witnessed Lucy giving her life for justice for others. But unfortunately we have not received justice for Lucy," he said. "We have been left feeling completely unresolved."

The lead judge, Tsutomu Tochigi, said there was "nothing to prove that the defendant was directly involved" in the rape and death of the 21-year-old former flight attendant, but acknowledged "suspicion", along with substantial circumstantial evidence linking Obara to the crime.

"The court cannot prove he was single-handedly involved in her death.

"What is clear is that the victim acted together with the accused and then vanished and following that, she was found dead."

The court heard that Obara - who denied all charges - went online to search for ways to dispose of a body in the summer of 2000 after Lucie disappeared from her job as a Tokyo nightclub hostess. He later visited a hardware store to buy concrete mix, a chainsaw, rope and tools. Staff at the store recalled Obara asking detailed questions about how to get rid of a large object and "whether concrete hardened in a cardboard box".

But the lack of DNA or other clinching evidence allowed Obara to escape the charge of killing her. Mr Blackman said the prosecution's failure to introduce evidence that he thought would have proved guilt had caused he and daughter Sophie "enormous anxiety and grief". But he said his decision last year to accept £420,000 from an intermediary of Obara's had no impact on the verdict.

"The judge made it very clear that he did not take into consideration at all condolence payments to victims," he said, adding that he was "very satisfied" with having taken the money. "After six and a half years of fighting for justice we would have walked away with absolutely nothing to help rebuild the family."

The verdict ends the brutal career of a man who will go down in history as Japan's most prolific modern serial rapist. Police believe that the multimillionaire property speculator may have targeted hundreds of women in a history of assaults going back to the early 1990s. The son of a Korean immigrant who had set up a chain of Pachinko, or pinball parlours, Obara made a fortune speculating in real estate during the years of Japan's bubble economy in the late 1980s and the early 1990s.

Driving expensive sports cars and permanently sporting a set of oversized sunglasses, Obara cruised the nightclubs of Tokyo in search of sex, preferring blonde foreign hostesses.

After luring victims back to his flat, according to prosecutors, he drugged them, removed their clothes, donned a mask and had sex in front of a video camera. Police found thousands of video tapes when they broke into the apartment, along with journals in which he recorded his ambition to sleep with 500 women "by the time I am 30". In other entries, Obara said that he was incapable of having sex with women who were conscious.

One of the pieces of evidence that helped convict him was a scrawled note beside Ms Ridgeway's name saying "Too much chloroform," along with a receipt for a hospital visit Obara made when he sought treatment for the seriously ill Australian, in 1992.

Last year, Tim Blackman called Obara a "monster" who had shown "not a single tear of contrition, shame or guilt for the perversion or crime against humanity". But despite prosecution claims that the killer could be out on parole within seven years, Mr Blackman said that it was "highly unlikely that Obara would see the light of day again".

Japanese justice

* Japan has one of the world's highest conviction rates, with more than 99 per cent of all defendants convicted. Confessions are still relied on heavily and failure to admit a crime is frowned on, making Obara (who denied his guilt right to the end) very unusual. While Japan incarcerates its citizens at less than half the rate of Britain, prison time is notoriously harsh. Inmates are kept isolated and mostly in silence, and forced to obey hundreds of military-like rules. Strip searches are common, as are beatings. One report in 2004 said there had been nearly 250 suspicious deaths in Japanese prisons in the previous decade. Police say Obara is unlikely to be released unless he becomes terminally ill in prison. "In Japan a life sentence is a life sentence," said one officer.

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