A notorious Russian serial killer known “The Werewolf” – who is already serving a life sentence for 22 murders – may be behind up to 37 more deaths, according to investigators digging into his crimes.

Mikhail Popkov, a 52-year-old former policeman from the city of Angarsk, north of Russia's border with Mongolia, reportedly told investigators that his kill total was 59.

Angarsk serial killer dubbed 'werewolf' got the life sentence for murdering 22 women. http://t.co/4O5sgNexLE pic.twitter.com/Q6MFFg01V4 — The Siberian Times (@siberian_times) January 15, 2015

He was already sentenced and convicted for 22 deaths and is awaiting trial for an additional 25 – while authorities told The Siberian Times Tuesday that they are looking into an additional 12 cases in which he may have been involved.

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“The investigation will be very long because there are a lot of cases,” senior investigator Andrei Bunayev told the news website last year. “He names the places where bodies are hidden. We find these bodies, and check his involvement.”

Bunayev said investigators were re-examining biological traces that were previously overlooked in some cases.

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Law enforcement sources told The Times it was likely Popkov’s total would pass those of other notorious Russian serial killers such as “The Butcher of Rostov” Andrei Chikatilo, who killed 52, and Alexander Pichushkin, the “Chessboard Killer”, who murdered 48.

In Popkov’s murders, he would sexually attack young female victims before killing them with axes, knives or screwdrivers, the website reported. Popkov would often offer victims rides in his police car before taking them to remote areas and raping and killing them.

“I admit my guilt in full... committing the murders, I was guided by my inner convictions,” Popkov previously told a court.

After being detained in 2012, he told authorities that he wanted to cleanse Angarsk’s streets of prostitutes.

The killer said he started the string of murders after suspecting that his wife was cheating on him when he found two used condoms in their home’s trash can.

But the condoms actually belonged to house guests, The Siberian Times reported.

“I just had some reasons to suspect her,” Popkov said. “I'm not looking for excuses, but this was the impetus for my future.”

Two of his victims were Tatiana Martynova, 20, and Yulia Kuprikova, 19, who were found dead in Angarsk after a night out in 1998.

“The pain does not go away - it was me who gave Tanya a ticket to go to a concert, and she was killed after attending it,” her sister Viktoria Chagaeva, told The Siberian Times.