Japanese police have arrested a woman and her parents over the beheading of a man in the city of Sapporo.

Hokkaido police said they arrested Runa Tamura, 29, and her father Osamu Tamura, a 59-year-old psychiatrist, on suspicion of conspiring in beheading the victim at a hotel room and relocating his severed head in the middle of the night between July 1 and July 2.

The head of the victim, Hitoshi Ura, 62, has been missing since then.

Police raided the suspects’ home on Tuesday and arrested the prime suspects’ mother Hiroko Tamura, a 60-year-old part-time worker, on suspicion of conspiring with her family in transporting and keeping the head at home.

Police did not say exactly how the daughter and the father collaborated. Police are still investigating the motive and refused to say if the woman and the victim knew each other.

Police also noted that Runa Tamura is a possible mental patient. Media reports quoted neighbours as saying that she has had difficulty attending school and had been reclusive since childhood.

Kyodo News and other media reported the victim and another individual believed to be Runa Tamura checked into the hotel in the Susukino area which is known for short-stay “love hotels”, About three hours later only one of them was seen leaving, carrying a large suitcase.

The person accompanying the victim was wearing light-coloured women’s clothing and a wide-brimmed hat when entering the hotel but was dressed in black when leaving, Kyodo said, quoting unnamed investigative sources.

Mr Ura’s body was discovered later on July 2 by a hotel worker who went to the room because no one had checked out by the afternoon.

The worker found the victim slumped in a bathtub, according to news reports. None of his belongings had been left in the room and the bed appeared unused.