Stephen Port murdered four young men during a 16-month period between 2014 and 2015, luring them to his one-bed flat in Barking before fatally plying them with date-rape drug GHB and then dumping their bodies nearby.
Port was initially arrested days after he killed his first victim, but was not charged with murder until after he struck for a fourth time.
Police said they were inexperienced to deal with such cases and were struggling with a large workload at the time of the deaths.
Here is what happened in each investigation:
Anthony Walgate, June 19 2014
It was Port himself who called 999 to report “finding” Mr Walgate unconscious outside his flat, although he later admitted dumping his body after the pair met for sex. He killed three men by the time he was jailed in March 2015 for perverting the course of justice in the Walgate case.
Fashion-conscious Mr Walgate was found with his underpants on back to front and inside out, but police said this was “unusual, not necessarily suspicious or sinister”.
Investigators at the scene were not told of a previous allegation of rape against Port on the police national computer from 2012, who was traced as being the person who called 999 to report the body.
Another officer deduced there was “nothing to suggest the victim had been assaulted”, despite bruising under Mr Walgate’s arms, while sex swabs taken from his body were not submitted for DNA testing.
Perhaps most crucially, police failed to submit a laptop belonging to Port for forensic analysis for 10 months after it was seized in the wake of Mr Walgate’s death, and then missed repeated searches for drug rape videos contained on the device.
The Crown Prosecution Service was also not given information that Port was a suspected sex offender when they ruled out a homicide charge over Mr Walgate’s death.
Police soon knew that Port had lied to them from the outset, eventually charging him with perverting the course of justice. But they did not follow up information he volunteered during his initial interviews about his previous involvement with police on suspicion of sexual assault.
Requests from the local police officers for the Met’s specialist murder investigation team to take over the investigation were repeatedly dismissed, meaning inexperienced officers were in charge of the case.
Two of Mr Walgate’s friends were convinced his death was suspicious and aired their concerns, but said they believed police assumed he had overdosed on drugs simply because he was a gay sex worker.
Sarah Sak, the victim’s mother, told police she was convinced it was “murder”, but said police told her “it was probably drugs”.
Gabriel Kovari, August 28 2014
Aspiring English teacher Mr Kovari was found slumped in a graveyard near Port’s home. But the scene was not declared suspicious, so his body was not subjected to a forensic post-mortem examination.
His clothing was not seized and so was not analysed, and no samples were taken from his body.
Police did not follow up leads to trace Mr Kovari’s friend, Karl Kamgdon, who Mr Kovari sent pictures to from inside Port’s flat when he arrived there, and was the last person he spoke to.
John Pape, Mr Kovari’s friend, effectively turned detective to find the victim’s boyfriend in Spain, Thierry Amodio, who had been contacted by Port masquerading as another man.
A family liaison officer assigned to the Kovari case failed to contact the victims’ loved ones at all, and even referred to him as being from Lithuania rather than Slovakia.
Daniel Whitworth, September 20 2014
Detectives failed to carry out key forensic tests including on the bed sheet on which chef Mr Whitworth was found, his clothes, so-called sex swabs taken from his body, and the drugs bottle planted on him – all of which carried Port’s DNA or fingerprints.
Perhaps most crucially, the “suicide note” planted by Port on Mr Whitworth’s body taking responsibility for Mr Kovari’s death appeared to be taken on face value by police.
Only a section of its contents was sent to Mr Whitworth’s family, with disputes over whether his father was able to identify the handwriting as that of his son.
The note was not shown to Mr Whitworth’s long-term boyfriend, Ricky Waumsley, for a year, and he said he felt cut out of the investigation entirely.
The note also contained a veiled reference to Port, asking police not to “blame the guy I was with last night” in case the investigation eventually linked the victim with the serial killer. However, police arriving on the scene thought “the guy” was a homeless man sleeping in the graveyard where Mr Whitworth was found.
It was much later that a handwriting expert concluded the script matched Port’s while the notepad on which the suicide note was written was recovered from Port’s address.
There was no evidence Mr Kovari and Mr Whitworth had ever met or contacted each other.
Police also failed to obtain full phone data that would have shown Mr Whitworth was not in Barking on the night of Mr Kovari’s death – creating an inaccuracy with the suicide note that claimed Mr Whitworth killed Mr Kovari.
Mr Walgate also showed signs of bruising under the arms consistent with being moved or carried before or after his death.
Bizarrely, dog walker Barbara Denham, who discovered Mr Kovari’s body, was also the first on the scene for Mr Whitworth’s death.
She told police: “I was the same woman that found the other body a few weeks ago … I found another young boy.”
Jack Taylor, September 14 2015
The final death was not linked to the previous three until a chance discovery nearly a month later – this was despite similarities that all four victims were young, gay men, with no links to the area, who were found dumped in public, within a short distance of each other.
Mr Taylor’s family shared with police their concerns that the death might be linked to others in the area, as did the coroner.
In fact, his sisters kept notes of their own investigations, establishing similarities between the four deaths, referencing GHB and identifying Port’s address in Cooke Street.
But their suggestions were dismissed by police who said there was “no reason to think they are connected”.
It was only when a detective working on the Walgate investigation chanced upon a print-out of a CCTV still of Mr Taylor with a “mystery man” on the night he was last seen alive that he recognised him as Port and made the link with Mr Walgate’s death.
Another officer then linked them with the Kovari and Whitworth investigations.