NOTHING could have prepared George Sanderson for the scene that greeted him at his daughter Claire's home.

Concerned about the welfare of his two daughters, the 56-year-old had travelled to the flat in Camblesforth, with Diane's boyfriend, Ian Harrison, on the morning of Sunday, July 18.

Arriving at about 8am, the pair were disturbed to find the door to the property unlocked and, walking into the flat, they were hit by an overpowering smell.

Hobson was nowhere to be seen.

Lying on the floor of the bedroom were the naked bodies of Mr Sanderson's beautiful, twin daughters, with their corpses covered in black bin liners.

Claire had been dead for several days, and her body had been wrapped in several bin liners and a plastic bag was placed over her head.

A post-mortem examination later revealed she had suffered several blows to the head with a hammer.

If not already dead from blows to the head, the plastic bag would have been fatal and caused her to suffocate.

A forensic examination of the flat, which had been partly washed and cleaned with bleach by Hobson, showed Claire had been attacked in the living room before she was dragged bleeding into the bedroom.

Lying nearby was Diane.

She was naked and lying on the floor on top of a large plastic bag.

Hobson had subjected the 27-year-old to a violent sexual attack, using an object to inflict severe injuries to her genital area.

The twin also had ligature marks to her wrists, her pubic area had been shaved and her left nipple was missing. Hobson had bitten it off.

Tests later revealed Diane had been subject to a sustained attack and had received several blows to the head before she was strangled with a ligature.

Like her sister, a plastic bag had been placed over her head.

Detective Superintendent Javad Ali said of the sexual assault: "It was absolutely horrific."

There was no evidence Claire had been sexually attacked.

At 8.09am, police were called, but little did they realise that, four hours later, they would also be dealing with two more horrific murders, committed by the same man.

The night before the grim discovery, Hobson had lured Diane to the flat. The shop worker was at home when she received a phone call from him.

Minutes later, she left the house, telling her parents she was going to call and see Claire before she met Ian in the pub.

She left her home at 7.15pm and was not seen alive again.

Ian had become concerned about her after she failed to meet him in the pub the night before. At about 9pm, he had called her mobile, but Hobson answered and said Mr Sanderson had suffered a heart attack and the girls were upset.

In reality, they were both dead.

Hobson agreed to meet Ian in the pub, where they stayed for a few drinks before they returned to the Camblesforth flat. Ian noticed a bad smell, but Hobson explained it away and said there was a problem with the drains.

Blood discovered on the settee was also explained away by the killer, who said Claire had been having women's problems.

Unknown to Ian as he sat talking to his friend in the living room, his girlfriend and her sister were lying dead only feet away.

The pair talked for some time before Ian left in the early hours, and Hobson made his way to the nearby home of his mother, Sandra.

Again, he came up with more elaborate lies. He told his mother that Diane and Claire had been run over and were in York District Hospital and she agreed to drive him there.

When they arrived, Hobson went in the hospital -he was caught on security camera at 2am -before he returned and told his mother he was going to stay with the girls. Police do not know why Hobson lied or why he went to York.

Detectives later discovered the former binman had broken into a bungalow in the York area. Fortunately, the occupants were out. A cigarette butt found at the scene linked him to the break-in.

At 8.15am that day, a neighbour of elderly Joan and James Britton left them safe and well, before his wife returned later to check on them.

Their back door was always left open. At 11.15am, the neighbour's wife went to see the Brittons but, to her horror, she found them dead.

Mr Britton had been attacked first. He had been beaten about the head several times with a walking stick and had been stabbed in the chest.

His frail wife died from a stab wound, which entered her back and had gone through several organs, including her stomach and liver.

The blade was still in her body, the handle broken off.

She had also been beaten about the head with a walking stick.

Forensic inquiries revealed both had been stabbed in the same manner.

Hobson then armed himself with a knife taken from the couple's kitchen and went on the run.

By 11am, police knew they had two double murders, but it was some time later before they were able to make a link after fingerprints matching Hobson's were found on shoeboxes in the Britton's bedroom.

His fingerprints were also found on a door.

When he was captured, Hobson said he could not remember carrying out the attacks on the pensioners.

No link between them has been discovered. He did not know the pair and had never worked for them,

Officers later found evidence in Hobson's flat that confirmed that killings had been pre-planned and that he also intended to kill more people.

Det Supt Ali said: "The case of Mark Hobson is a story of a cruel and callous killer, it is a story of four brutal and savage killings, a story of four totally unnecessary deaths.

"We do not know what Mark Hobson's motives were or why he carried out the four killings."