Takeshi Koizumi, 46, was said to bear a grudge over the death of his pet at a health centre, but investigators said they were treating his confession with caution.

Japan, which has one of the world's lowest crime rates, was shocked after the discovery on Tuesday of the dead bodies of a 61-year-old former vice-welfare minister and his wife at their home in Saitama, north of Tokyo.

'I killed the (former) vice-minister,' Koizumi was quoted as telling a police officer when he turned himself in.

A police spokesman said he was arrested on suspicion of violating the swords control law, which bans possessing or carrying knives without permission or proper reasons, but that he had not been arrested for the murders as yet.

'A knife with a 20-centimetre blade was found in the car he drove here,' the spokesman said.

Media reports said the knife was bloodstained.

The wife of another 76-year-old former vice-welfare minister was seriously wounded on her doorstep late Tuesday, attacked by a man pretending to be from a parcel delivery service.

Both officials once served as directors at the welfare ministry's troubled pensions division, raising the possibility that the attacks are linked to the loss of millions of pension payment records.

Koizumi also admitted he had attacked the wives, reports said.

Japanese media also aired footage of Koizumi's father, who said 'I never dreamed my child might do this kind of thing.'

The father said he received a telephone call from his son on Saturday, hours before Koizumi turned himself in to police, after a decade of no contact.

'He told me that he sent me a letter. As he didn't sound strange and was actually speaking cheerfully, I thought he may have got a wife,' the father said. -- AFP