TOKYO - A MAN who confessed to the stabbing death of a former health official sent an e-mail, published on Monday in Japanese newspapers, that accuses the ministry of killing his childhood pet dog.

In the e-mail, which Takeshi Koizumi apparently sent to several media outlets before turning himself in, he says he was not seeking revenge for the Health Ministry's mishandling of millions of pension records - a motive that police had initially put forward and say they are still pursuing.

Police have been also investigating whether the crime is related to the stabbing of another former health official's wife. Both bureaucrats were instrumental in setting up the pension system, and its mishandling stripped many Japanese of their retirement funds, causing widespread public anger.

Koizumi, 46, was arrested early on Sunday for illegal possession of weapon - a bloodstained knife found in the back of a rented car he drove to the Tokyo police headquarters. The former vice minister was stabbed to death along with his wife last week. Hours later another bureaucrat's wife was attacked but survived.

A Tokyo police official said investigators are preparing to add murder charges against the man in the high-profile case.

Media reports have said Koizumi told police he blames the Health Ministry for the death of his own dog, but police have not confirmed that report.

The suspect's father told public broadcaster NHK and other media outlets that the family had adopted a stray dog when Koizumi was a child but took it to a local public health department to have it put down because it barked constantly.

'It was revenge for the killing by a healthcare centre of my family 34 years ago,' said the e-mail message published on Monday by mainstream newspapers, including the Yomiuri.

'I killed one monster (former bureaucrat) and a coarse fish (those living with the monster), but even now, they continue killing as many as 500,000 innocent pets every year.'

Mr Takehiko Yamaguchi, 66, and his wife Michiko, 61, were found dead last week in their home outside Tokyo. Hours later, the 72-year-old wife of another vice-health minister was stabbed at their Tokyo home but survived the attack with serious injuries. Ms Yasuko Yoshihara's husband, Kenji, was not home.

Police have only confirmed that Koizumi confessed to killing Mr Takehiko Yamaguchi, but media reports have said he has confessed to attacking all three people last week. -- AP