German nurse Niels Hoegel is on trial for at least 99 murders, having already been jailed for six, which would make him among the worst serial killers in recent history.
Here are seven others:
'The Chessboard Killer'
Russia's Alexander Pichushkin, then aged 33, was sentenced in 2007 to life in prison for 48 murders, most between 2002 and 2006.
His victims were mainly elderly alcoholic men whom he met in a park outside Moscow.
He threw them into a sewer where they drowned, later strangling or bludgeoning them with a hammer first after some managed to climb out. He threw one from a 16th floor.
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.
Yang Xinhai was executed in China in 2004, aged 35, after murdering 67 people in a three-year rampage that ended with his capture in 2003.
Characterised as an introverted drifter, he bicycled from one murder scene to the next, entering homes in rural areas and often slaughtering entire families using an axe, hammer or spade.
Police listed robbery aa one motive, but Yang was also described as a deranged killer who simply enjoyed what he was doing.
Gary Ridgway, a former truck painter, was handed 48 life sentences in 2003 after admitting to killing 48 prostitutes and runaways between 1982 and 1984, a murder spree that terrified Seattle. He was 54 years old at his conviction.
Nicknamed the "Green River Killer" after the waterway where his first five victims were found, he is the most prolific serial killer in US history.
A family doctor in northern England, Harold Shipman was sentenced to life in prison 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 the following year found that Shipman, nicknamed "Doctor Death", had killed around 250 patients between 1971 and 1998, which made him the country's worst-ever serial killer.
Luis Alfredo Garavito, a travelling salesman, was jailed for 835 years in 2000 aged 42 for murdering 189 boys aged between eight and 16 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.
He lured them with food and drink and usually slit the children's throat; some were beheaded.
Four of the murders took place in Ecuador.
In 1992 Russian Andrei Chikatilo, 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 far-eastern area where he was particularly active, was executed in 1994.
'Monster of the Andes'
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.
Initially accused of murdering four children, the "Monster of the Andes" led police to a field where the bodies of 53 young girls were buried.
He was sentenced to 16 years in prison, the maximum term in Ecuador, and 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.
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