DETECTIVES remain baffled over why Hobson killed and killed again.

"The mystery still remains. We don't know what Mark Hobson's motives were and why he carried out four killings," said Detective Superintendent Javad Ali, who led the investigation.

But police knew there had been four "totally unneccesary deaths" at the hands of a cruel and callous killer.

They had also found evidence that at least some of the murders were premeditated.

Det Supt Ali said hand-written notes had been found in Hobson's flat, containing a "shopping list" for items including bin liners, tie wraps, fly spray and air freshener.

He said these were either to be used in the killings or to deal with the consequences of having bodies in the flat.

The detective said Hobson and Claire Sanderson had been together for about 18 months, living in a flat in Camblesforth, near Selby.

Claire was last seen at 7.30pm on July 10, at the Comus Inn in the village. Her sister, Diane, was last seen at 6.45pm on July 17, when she told her parents she was going to see Claire at the flat. "Hobson had lured her to her death," he said.

Diane's boyfriend, Ian Harrison, was waiting for Claire in the Cricketers Arms, in Selby, but she never turned up. At 9pm, he tried to ring her mobile, but it was answered by Hobson, who told Harrison the twins' father had died of a heart attack and they were so upset they didn't want to talk.

Hobson joined Harrison at the pub for a drink, but then invited him back to the flat.

Harrison noticed an unpleasant smell there, but Hobson blamed it on blocked drains. Harrison noticed blood on a settee but Hobson explained this away by saying Claire had had "women's problems."

Harrison left in the early hours, concerned about Diane.

Hobson then gave his mother a "cock and bull story" that Claire and Diane had been injured in a road accident, and she agreed to give him a lift to York Hospital.

He was captured on CCTV footage walking into the hospital at about 2am. He later came out and told his waiting mother the girls had been kept in hospital, and he would wait until Claire was discharged, and his mother went home.

Hobson then walked to New Lane, Huntington, where he attempted to break into a bungalow. Tests on a cigarette end found at the scene showed it had been smoked by him.

Meanwhile, at 7am, Harrison, concerned about Diane, went round to her father's house, where Mr Sanderson was obviously safe and well.

They both went round to Hobson's flat, where there was a terrible smell of rotting flesh and they found Claire's body in an advanced state of decomposition, with her head in a black bin bag.

A post mortem examination revealed she had suffered several blows to the head with a hammer. It was not clear if it was the blows or the bag over the head which had proved fatal. Attempts had been made to clean the body with bleach.

Diane's body was found with ligature marks round her neck, ankles and wrist, and she had been sexually mutilated. She had also been strangled.

At this time, Hobson was making his way to Strensall, where James and Joan Britton, a frail and elderly couple, lived in a house on the edge of the village.

Mr Britton, 80, suffered from a condition similar to Parkinson's Disease, deafness and memory loss, and Mrs Britton was even more frail and walked with a frame on wheels.

At 8.55am, a neighbour who kept an eye on the couple saw them safe and well. At 11.15am, a neighbour found the couple dead.

Mr Britton had been stabbed in the back and beaten, and his wife had also been stabbed in the back. The blade was still embedded in her back. She had also suffered blows to the head causing severe injuries.

"By 11.30am, we knew were dealing with two double homicides," said DS Ali.

Updated: 13:53 Tuesday, April 19, 2005