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The Yorkshire Ripper was a necrophiliac who masturbated over corpses and wore silk “murder” trousers to get his sexual kicks, an author has claimed.

Peter Sutcliffe murdered 13 women between 1975 and 1980 in Leeds and Bradford, west Yorks.

Author Richard Charles Cobb now claims to have pinpointed where each of his victims were killed in new book On the Trail of the Yorkshire Ripper: His Final Secrets Revealed.

Mr Cobb believes he's also worked out the real motive for Sutcliffe's heinous crimes.

He said: "Most people I spoke to say he thought God told him to kill prostitutes.

"That claim is nonsense when you consider the people he attacked – who didn't die – were not prostitutes."

After carrying out extensive interviews, Mr Cobb believes that Sutcliffe was a necrophiliac who murdered his victims to get sexual pleasure.

The investigator believes this started when he worked as a gravedigger.

"There were all sorts of quirky morbid stories about how he would open the coffins and look at the bodies when he worked at the mortuary," Mr Cobb explained.

"During the time when he worked as a gravedigger, he was getting sexual kicks from the bodies."

(Image: Richard Cobb/Triangle News)

The investigator and his team also think Sutcliffe's "murder trousers" show he was a twisted deviant.

He fashioned pants out of a green silk shirt, adding kneepads on the elbow and a hole in the crotch area.

"This was designed to kneel over the body exposing himself, and pleasure himself over the body," the author said.

"After he lost his job as a gravedigger I think he wanted to find a way of regaining that sexual high.

"And that's when I think he started going out and attacking prostitutes.

"Hitting them on the back of the head immobilises them, and then they're lying on the ground resembling a dead body.

(Image: Getty Images)

"Sutcliffe then performs his sexual deeds, and he kills them with a knife."

Semen stains were found on the clothes of Sutcliffe's first victim Wilma McCann in 1975.

"Everyone has always wondered a motive for the crimes, that's the motive – he's a necrophiliac," Mr Cobb added.

He also has a new theory about the murder of Marguerite Walls – the only killing Sutcliffe didn't confess to.

Marguerite was killed in the posh Leeds suburb of Farley in 1980.

Sutcliffe says he thought she was a prostitute, so decided to attack and murder her.

But Mr Cobb believes he was in fact waiting to kill Maureen Long, the only victim who lived to tell the tale.

(Image: Daily Star)

She lived on the other side of the road of Ms Walls, and was the only eyewitness to Sutcliffe's heinous crimes.

"Why he didn't confess to Ms Walls' murder, because when he was arrested he was going to go for the insanity plea," Mr Cobb said.

"He said he was on a mission to kill prostitutes only.

"If he was found premeditating to kill a witness, who wasn't a prostitute, it would probably destroy his claims in court.

"When the police didn't link up the addresses, he freely admitted to killing her claiming he thought she was a prostitute.

"Ms Long was the only witness who could identify him, and I think he wanted to finish her off.

"Until this day no one has linked the attacks together."

And Mr Cobb believes this further refutes the "myth" that Sutcliffe only wanted to kill prostitutes.