The killer is now behind bars (Image: PA)
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A dangerous predator convicted of killing two women 21 years apart was revealed today as suspected serial killer.
Twisted Gary Allen, 47, had denied murdering Samantha Class in Hull in 1997 and Alena Grlakova in Rotherham in 2018.
He was cleared of the murder of Samantha Class following a trial in 2000 but this acquittal was later overturned.
Within weeks of being cleared, he had attacked and strangled two sex workers after moving to Plymouth.
Ms Class's body was found by children walking past the banks of the Humber in North Ferriby 24 years ago, the jury at Sheffield crown court heard.
She had been stamped on, strangled with a ligature and run over with a car before her body was dumped in the water in a 'brutal' murder.
Ms Grlakova's body was found naked in a stream in Rotherham in April 2019, after she was last seen on Boxing Day 2018
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Ms Class, who was 29, was had been a sex worker at the time of her death.
Ms Grlakova's body was found naked in a stream in Rotherham in April 2019, after she was last seen on Boxing Day 2018.
She was born and brought up in Slovakia and moved to the UK in 2008.
Police said she was at a "low point in her life" at the time of her death and had started to work in the sex industry.
In the aftermath of her death, sex workers posted warnings online about Allen, a violent predator with a grudge against women.
Police forces across the country will now be asked to look back at any unsolved murders to see if there are any links to Allen.
Speaking after the case, DCI Mark Oughton, who led the double murder inquiry for South Yorkshire Police, said: "He has killed two sex workers two decades apart, and we have already triggered a full review of this case.
"We have a full picture of his movements and sought advice from the National Crime Agency and their Serious Crime Analysis Section.
Alena Grlakova who was last seen on Boxing Day 2018
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"They keep a record of all serious sex crimes and murders and any potential links will be flagged up to relevant forces to investigate further."
He added: "You have heard in court his hatred of sex workers. His mobile phone showed deeply disturbing images of deceased females, downloaded from the web, some of them partially buried, including sex workers.
"The families of the victims have been at the forefront of our minds – we all wanted to get them justice."
The discovery of Ms Grlakova's body saw South Yorkshire Police embark on a huge investigation to build up evidence against Allen.
They took 528 witness statements, with a total of 880 people seen by 232 officers.
There were 1,096 exhibits seized along with 37,624 hours of CCTV.
Allen was found guilty of murdering the sex workers at Sheffield Crown Court
(Image: BPM MEDIA)
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A team of 30 officers was used for the inquiry. Prosecutor Alistair MacDonald QC said Allen was interviewed by probation officers in 2002 and told them about his hatred of women, particularly sex workers.
Allen told one probation officer: "I like to frighten them. I like to cause pain. I like to make them cry. I like blood. I like to hurt them. I enjoy it. It makes me feel good."
He was deemed to be so dangerous that he was at the centre of a two-year undercover operation involving a total of seven officers.
They posed as his friends, along with their partners and families.
Operation Misty' saw Allen confess to the murder of Ms Class to an officer known as 'Ian'.
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The recording of that confession was key to overturning not guilty verdict on Allen for Ms Class's murder in 2000.
Operation Misty began in 2010 when Allen returned to live in the Humberside Police area after being released from prison.
He was recorded in December 2010 telling Ian that a sex worker had threatened to report him for rape if he did not pay her "so I strangled her… and dumped her in the Humber".
Prosecutors said this was the "clearest possible admission to the defendant's guilt in murdering Samantha Class".
In Feb., 2011 Allen again referred to killing Ms Class, saying he "just went f ** mad".
Det Chief Insp Christine Calvert, who led the recent investigation for Humberside Police, said: "It was a two-year operation and they used seven different undercover officers. They all played varied parts. There were others on the periphery that were brought in to build up the story."
She went on: "Gary Allen was a loner. We obviously had highly-trained officers but they had to be really dedicated over a long period of time to keep that relationship going."
She said Ian did "a fantastic job and deserves every commendation going". Asked why Humberside Police were so determined to put Allen behind bars despite his 2000 acquittal, Ms Calvert said: "I think it was the risk he posed to the public over that period of time.
"It was just getting justice, really, for the family. Samantha's daughter was only 10 when it happened. All three of Samantha's children have grown up without a mother and it was getting that justice for them."
Ms Calvert said Ms Class's daughter had spent more than 20 years wondering where Allen was and what he was doing. "Now he's behind bars and, hopefully, for the rest of his life or for a very long time, that will be a little bit of closure for her," she said. "It's a fantastic result."
Mr Justice Goose will sentence Allen on Wednesday. The judge told Allen: "You should prepare yourself inevitably for a life sentence, I will determine the details of that next week at 2pm."