JOIN USEx-Russian policeman Mikhail Popkov, found guilty Monday of 56 murders while already in jail for 22, is one of the most prolific serial killers of recent times.
Here is a recap of some of the others, starting with two more serial killers from Russia.
Alexander Pichushkin
Alexander was sentenced to life in prison in Moscow in 2007 for 48 murders, most between 2002 and 2006.
Aged 33 at his trial, Pichushkin said he wanted to kill one person for each of the 64 squares on a chessboard, and crossed out a square for every kill, earning him his nickname.
His victims were mainly elderly alcoholic men whom he met in a park outside Moscow.
Andrei Chikatilo
In 1992 Russian Andrei, 56, was sentenced to death for 52 sexually motivated killings of women, children and young people between 1978 and 1990.
The former teacher, known as the "Butcher of Rostov" after the area in southern Russia where he was particularly active, was executed in 1994.
Samuel Little
A 78-year-old drifter in prison in Texas confessed in November 2018 to 90 murders and is being investigated as possibly the most prolific serial killer in US history.
Samuel Little preyed mainly on drug addicts and prostitutes during a decades-long murder spree, according to the Federal Bureau of Investigation (FBI).
Law enforcement has so far verified 34 killings.
Niels Hoegel
Former hospital nurse Niels Hoegel admitted at his trial in October 2018 to having killed 100 patients on top of six for whose murders he had already been convicted.
Hoegel, 41, confessed to giving patients drug overdoses because he enjoyed the thrill of trying to reanimate them at the last moment.
Investigators say the final toll could top 200.
Gary Ridgway
US truck painter Gary Ridgway confessed in 2003 to the murders of 48 prostitutes and runaways from 1982-1984 but he is suspected of around 90 killings.
Nicknamed the "Green River Killer" after the Seattle waterway where his first victims were found, he was 54 years old at his conviction and jailing.
Harold Shipman
Family doctor Harold Shipman was sentenced to life in 2000 after being convicted of killing 15 of his elderly patients by giving them fatal doses of morphine.
He hanged himself in prison in 2004, aged 57.
An inquiry found that Shipman, nicknamed "Doctor Death", had killed around 250 patients between 1971 and 1998, making him the country's worst-ever serial killer.
Luis Garavito
Luis Alfredo Garavito, a travelling salesman, was jailed for 835 years in 2000 aged 42 for murdering 189 boys over a five-year span until 1996.
Known as "The Monster of Genova" after his birthplace in Colombia, Garavito met his victims by posing as a charity worker, salesman, monk or disabled person, among other disguises.
Pedro Monsalve
In 1980 Colombian Pedro Lopez Monsalve was arrested at a market in Ecuador after attempting to abduct a young girl. He later confessed to having strangled at least 310 children from poor backgrounds in Colombia, Ecuador and Peru.
The "Monster of the Andes" was first sentenced to 16 years in prison, the maximum term in Ecuador, and then extradited in 1994 to Colombia where he was interned in a psychiatric hospital.
Freed several years later, he disappeared and would today be aged around 70.