Over 15 months, Stephen Port, a 41-year-old chef from a working class east London borough drugged, raped, and killed four gay men he met on dating apps.
He then dragged their bodies a few hundred meters from his flat, planting drugs and even suicide notes penned in his own handwriting to make it appear as if they had overdosed.
Three of the victims were found slumped in the same churchyard -- two of those discovered by dog-walker Barbara Denham.
A fourth victim was found at the entrance to Port's block of flats.
"The serial killers which are most difficult to catch are transient -- they kill in one part of the country, then another," explained David Wilson, professor of criminology at Birmingham City University.
"But Stephen Port was not a transient serial killer. He killed in his flat, then dumped the bodies 300 meters away. They were all young men who died the same way," he told CNN.
"The idea that it wasn't linked beggars belief."
Police under investigation
On Friday Port will be sentenced for the murders of fashion student Anthony Walgate, 23; Gabriel Kovari, 22, originally from Slovakia; fellow chef Daniel Whitworth, 21; and forklift truck driver Jack Taylor, 25 -- crimes that Port denied.
London's Metropolitan Police admitted there may have been "missed opportunities to catch Port sooner" during the spate of murders between June 2014 and September 2015.
The force is now reviewing the deaths of dozens of men who also used sex-enhancing drugs, amid concerns that cases previously dismissed as drug overdoses could be further victims.
The Independent Police Complaints Commission (IPCC) has also launched an investigation into police handling of the case.
What if the victims were women?
Leading gay rights activist Peter Tatchell questioned whether homophobia and class played a role in police handling of the case.
"If four young middle class women had been murdered in Mayfair, I believe the police would have made a public appeal much sooner and mounted a far more comprehensive investigation," he said, referring to a wealthy area of inner London.
"The killing of low income gay men in working class Barking was treated very differently -- police officers stand accused of class, gender and sexuality bias."
Criminologist Wilson agreed that the case had "a great deal to do with the persistence of homophobia generally and within the police."
However he added that one group of female victims similarly ignored by the police were sex workers.
"It was some way down the line before the continuing disappearance of women in Ipswich started to have an impact on public consciousness," he said of the 2006 Ipswich murders of five prostitutes by serial killer Steven Wright.
"One of the sad things is that everyone is equal before the law -- but the level of policing they get is often dependent on other issues."
Newspaper tip-off ignored?
When Benjamin Cohen, chief executive of LGBT newspaper Pink News, was contacted by a friend of victim Gabriel Kovari asking him to investigate a link between the Barking murders, he says police dissuaded him from pursuing the story.
"They told us there wasn't a link between what was at that point three deaths," Cohen told CNN. "Essentially, they gave the indication it would be a bad idea for us to report anything as it would cause people to be fearful.
"They were saying the deaths were not in suspicious circumstances," he added. "But if it were three young women murdered in Islington or Hampstead and they had an inquiry from the Daily Mail, I cannot believe that would happen," Cohen said of the high-end London boroughs.
Chillingly, it was Port who called the police pretending to be a bystander who spotted the body of his first murder victim Anthony Walgate. He was later arrested but only charged with perverting the course of justice and jailed for eight months.
Port went on to kill three more men, and Cohen expresses disbelief that police were able to let the killer slip through their fingers so easily.
"I can imagine Netflix making this into a 10-part series where the murderer just keeps getting ignored," he said.
'You don't have chemsex by yourself'
Prosecutors said Port targeted victims on Grindr and other gay dating sites and apps, then plied them with drinks laced with fatal amounts of the drug GHB, or gamma hydroxybutyrate, otherwise known as liquid ecstasy.
He then raped them while they were unconscious and before they died, according to testimonyat the Central Criminal Court of England and Wales, commonly known as the Old Bailey.
Port dragged the bodies not far from his home and planted drugs on or near their bodies to make it look as though they had overdosed.
Cohen believes another reason why police may have overlooked the serial killer's involvement, was "because of the use of the drug GHB and its prevalence among the gay community."
"It's quite a problem in the gay community in London, of people using these drugs in chemsex," he said of the group sex sessions often organized through dating apps.
GHB, along with crystal meth, is often associated with chemsex due to its ability to induce heightened arousal, sexual stamina, and reduce inhibition.
"But the thing that's odd to me is, you don't have chemsex by yourself, you have chemsex with someone else," said Cohen. "So if all of these people had been taking this drug, they must have been sleeping with someone to do it."
That person was a lot closer than they thought.