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One of the world’s worst serial killers has fathered a baby with a pen pal 40 years his junior.

Jailed ex-cop Sergey Tkach, 65, may have murdered more than 100 women and girls, including one aged only eight.

In a 25-year reign of terror, the cold-blooded psychopath had sex with the dead bodies then used his police training to hide his sickening crimes.

His wife Elena, who was not born when he started his killing spree, was a teenager when she saw the jailed monster in a TV ­interview and became besotted with him.

Elena, 25, sent him perfumed letters which led to prison visits and, in 2015, they wed. Tkach is yet to meet their daughter Elizaveta, born 19 months ago.

(Image: Netflix)

They have regular conjugal visits yet Elena does not fear for her safety, even though Tkach has access to kitchen knives in the ­apartment they use.

She is equally unconcerned about her husband’s victims. In a shocking interview she revealed: “I don’t feel guilty because I did not commit the crimes. Of course I’d not wish this on anyone and if it happened to my family I’d punish that person.

“But I didn’t do anything.”

The killer was jailed in 2008 in Ukraine, where there is no death penalty. She said: “When I saw his interview there was a sort of ­magnetism. I wanted to have a conversation, get to know him. I wanted to make him like me.”

At first they spoke only by phone ­separated by a glass panel but officials then allowed them to be in the same room.

She said: “I felt happiness when I ­realised I was pregnant. I wanted this baby and he wanted this baby but she was born in Russia and my parents and the officials won’t let me bring her to this country.”

Elena’s horrified parents are said to have pleaded with the courts to strip her of parental rights for Elizaveta.

Meanwhile Elena coolly campaigns for the release of Tkach, who is caged with dozens of rapists and murders in the tough Zhytomyr Prison, a former Gestapo jail, in north west Ukraine.

Three times-divorced Russian Tkach was sacked from the Soviet police in 1979 for falsifying evidence. He began preying on girls across Ukraine and Crimea, ­typically strangling them before violating their dead bodies. He evaded capture for a quarter of a century and was finally snared in 2005 – after brazenly attending the funeral of one of his young victims.

Three years later he was convicted of 37 murders, though he reportedly ­boasted he’d killed scores more children and teenagers. When first convicted he is said to have begged for the death penalty.

But now he says: “I want to live for the love of my daughter and my wife.” His astonishing marriage is laid bare to UK audiences for the first time in an episode of a shocking new Netflix series, Inside The World’s Toughest Prisons.

The documentary is presented by ­journalist Raphael Rowe, who was freed after serving 12 years in a British jail for a murder following the quashing of his conviction by the Court of Appeal.

Rowe says fiercely jealous Elena ­insisted an ­all-male crew visit the couple in the cosy apartment where they enjoy marathon sex sessions.

Criminals such as Tkach are usually held in cages but Ukrainian laws mean all married prisoners are allowed a three-day conjugal visit every two months.

Elena previously told Ukrainian media: “If I had to deal with his previous wives they wouldn’t be walking this earth any more. I wouldn’t let anyone within a mile of him. I’m a very jealous woman.

“I’m not scared by his conviction. Good women are scared of him. If that’s the case, there is less competition for me.”

Rowe said: “She made it clear to me and my team we were not to bring another female to the interview for him to ogle and lust over. She dominated the ­conversation. It was strange. She seemed to relish the attention he gave her.”

(Image: Netflix)

In bizarre scenes, the couple snog in the prison flat which looks like any normal marital home. Tkach boasts: “Today we can hug and kiss freely.” He then makes a joke about eating glamorous Elena’s lipstick.

But when alone in his cage he shrugs off his crimes and chillingly tells Rowe: “Women are guilty of everything. If I killed 30, so be it. I did not appeal the verdict.”

Guards check on Elena almost hourly to ensure she’s still alive in the compound, where Sergey has unrestricted access to kitchen knives. Prison psychologists are said to be terrified she would not live if she ever split up from the brute.

(Image: Netflix)

But Elena says she is not scared of having sex with him. And she disputes claims he sent more than 100 people to early graves.

She said: “I don’t fear anything negative. I don’t believe the total. I believe it was a lot less.”

Local media reports that Elena is desperate for three kids by the ­monster, who claims he still has a “strong seed” despite his age.

Deluded Elena is reported to be confident that Tkach, who has four children from previous ­marriages, will one day be released.

She is said to have bragged that they will build a home on the shores of Lake Baikal in eastern Russia.

Elena said: “His age, health and the presence of children could serve in his favour. I love him. I’m sure ­everything will work out for us.

“He’s the harmless one. I’m the dangerous one.”