Ariyama's body was found the same evening, missing some teeth that Kobayashi had wrenched out after killing her.

Nearly a month later, as the hunt for the killer went on, Kobayashi sent another e-mail on the mobile phone to the girl's mother, saying, "I'll take her baby sister next".

Kobayashi's reaction to the news that he would be hanged is not known, as his legal team and family were informed that the sentence had been carried out later in the day.

Two other death-row prisoners were hanged the same day. Masahiro Kanagawa, aged 29, had been found guilty of killing one man and injuring seven others in a random knife spree in Ibaraki prefecture in 2008, while 62-year-old Keiki Kano was executed for the murder of a bar owner in Nagoya in March 2002.

At a news conference that day, Justice Minister Sadakazu Tanigaki said: "All these cases involved atrocious crimes that stole precious lives for selfish reasons."

Questioned by reporters, Tanigaki declined to comment on the timing of the executions or why the three men had been chosen from the 130 on death row.

But opponents of the death penalty believe they know why Tanigaki, part of the Liberal Democratic Party that swept back into power in December, signed the execution orders.

"It took the Democratic Party of Japan administration more than a year to make a decision to carry out its first execution, but the LDP has carried out its first executions less than two months after being elected," said Yoshihiro Yasuda, a lawyer and a member of Forum 90, one of a coalition of groups demanding that Japan abolish the death penalty.

"This administration will have no hesitation in carrying out executions and we expect them to take a proactive approach to carrying out the death penalty," Yasuda added.

The LDP's "bold approach" to the death penalty was at odds with global trends, he added, with the number of countries that retain the punishment shrinking to just 20. Of the member states of the Organisation for Economic Co-operation and Development and the G-8, only Japan and the United States still have the death penalty on their statute books.

In the face of international pressure to do away with capital punishment, the Japanese government considers the death penalty to be an internal matter and popular with the electorate.

In the most recent public opinion poll conducted by the government, fully 85 per cent of Japanese said they approved of capital punishment - although opponents dispute this figure.

"The survey is filled with leading questions," said Hideki Wakabayashi, secretary general of Amnesty International Japan.

"I doubt very much if the results really reflect the will of the people."

Wakabayashi believes that closer to half of Japanese support the death penalty, although that is still a substantial figure. He believes that there is an urgent need for the public to debate the issue instead of being fed the government line - thanks in large part to a compliant media that prefers to play up the lurid details of the crimes instead of considering the pros and cons of hanging.

"The Japanese government should immediately suspend the death penalty and engage in debate with the public," he said. "That debate should be conducted on the principle of human rights, the most fundamental of which is the right to life."

In a public appeal to the minister of justice, the groups opposed to capital punishment describe it as "a cruel and inhumane penalty bereft of any human respect and which robs people of their fundamental right to life". They also point out retaining the death penalty makes a mockery of Japan's ratification of numerous international treaties on human and civil rights.

And while the opponents of hanging are fundamentally against the state exercising the right to take a citizen's life, they also express particular concern over a number of cases in Japan in which the death penalty has been handed down to a defendant despite evidence of a miscarriage of justice.

Yasuda said there were three particularly worrying cases in which death penalties had been passed - including one in which the sentence had already been carried out.

In February 1992, two primary schoolgirls disappeared on their way home in Iizuka city, Fukuoka prefecture. They were found dead the following day.

Two years later, police arrested Michitoshi Kuma on the grounds that he drove a car similar to one seen close to where the girls were abducted, despite lawyers pointing out that 127 vehicles fitting the same description were registered in the prefecture.

Kuma consistently and vigorously proclaimed his innocence but, in the absence of any direct evidence linking him to the crime, the court decided that the blood and DNA evidence on the bodies of the victims matched that of Kuma and convicted him.

That sentence was finalised by the Supreme Court in 2006 and Kuma was executed on October 28, 2008, still proclaiming his innocence.

The hanging also went ahead just two weeks after the Tokyo High Court prosecutors' office issued a written opinion that DNA evidence used in another murder case was inaccurate and could not be used. Exactly the same method had been used to secure the DNA evidence that convicted Kuma.

In 2009, Kuma's family filed a request for a posthumous retrial and, when the case came to court in 2012, the defence team told the judge that a new DNA test confirmed that the samples did not belong to the victims thought to be those of Kuma.

The defence also presented evidence that part of the DNA images that were presented to the court by the prosecutors had been cut out, the part showing the DNA of the third party.

It is Yasuda's contention that the evidence was tampered with, and the defence demanded that all testing records, photographs and other evidence be provided for scrutiny. The prosecutors claim no such materials exist.

Another famous case in which the actions of the police are under scrutiny concerns Iwao Hakamada and dates back as far as June 1966.

The Hakamada case involved the murder of a businessman in Shizuoka prefecture and three of his relatives. After 23 days of near-constant interrogation, Hakamada "confessed".

Despite withdrawing his confession in court and telling the judge he was given no food or water, was beaten and kicked and allowed to speak with his three lawyers only on three brief occasions, he was sentenced to death.

Now 77, Hakamada has been incarcerated for 46 years.

Despite repeated setbacks in his legal team's quest for a retrial, a motion to re-examine the case is under way. The appeal has been bolstered by prosecutors finally admitting that audio tapes of the interrogation do exist, while there are also suggestions that police fabricated evidence involving bloodstains on clothing Hakamada allegedly wore during the attacks.

"Despite repeated calls for a new trial and DNA tests that show Hakamada is probably not guilty, we are still waiting for a new trial to be set," Yasuda said.

But Yasuda believes the death-row inmate most at risk of execution is Shoko Asahara, the figurehead of his own religious movement who was convicted of 13 charges, including murder and masterminding the release of sarin nerve gas on the Tokyo underground system in 1995.

Asahara, the self-proclaimed guru of the religious sect Aum Shinrikyo, is a deeply unsympathetic person among Japanese, making him an easy target for the justice minister, Yasuda believes.

"In the media, there has been a movement calling for Asahara to be executed as soon as possible," said Yasuda, who was a member of Asahara's defence team. "I believe there is a great risk that Mr Tanigaki will respond to that by implementing the death penalty for Asahara.

"There are many people saying that if he goes ahead and signs the execution order, then his political status will rise. I feel very strongly that ordering the death penalty for political gain should never happen."

And while he accepts that Asahara was responsible for the atrocious acts carried out in the name of Aum Shinrikyo, he also believes that he is in no fit state to stand trial, let alone be executed.

"When I last met him, many years ago now, I could tell that he was not in a fit mental state and a number of psychiatrists agreed with that assessment," Yasuda said. "Despite this, the authorities insisted that he was fit to stand trial.

"Actually, since June 2006, no one has been able to meet him, either his family or legal team, so we do not even know if he is alive or dead already."