Eight former and current Met Police officers are being investigated for gross misconduct regarding failings in the case of serial killer Stephen Port.
Anthony Walgate, Gabriel Kovari, Daniel Whitworth and Jack Taylor were all murdered between 2014 and 2015.
They were drugged with overdoses of GHB by Port, who dumped their bodies near his flat in Barking, east London.
The Independent Office for Police Conduct (IOPC) will examine alleged breaches of professional standards.
Port, who met his victims online, is serving a whole-life prison term after being convicted in 2016 at an Old Bailey trial.
After Port was jailed, the police watchdog took years to decide no officers should be disciplined.
But that conclusion was upended by inquests into the deaths in 2021, which laid bare multiple Met failures that a jury found contributed to three of the deaths.
Basic errors by a string of detectives left Port free to carry out the three murders, as well as to drug and sexually assault more than a dozen other men.
The watchdog was forced to reopen its inquiry and, two years on, has announced a new stage of its investigation.
The inquiry relates to equality and diversity, duties and responsibilities, authority, respect and courtesy, and honesty and integrity.
Five of the eight people under investigation are serving officers.
'Live this nightmare'
Sisters of Jack Taylor, Donna and Jenny, said in a statement: "The news today made us feel grateful that someone is finally taking this serious and that our Jack and also Anthony, Gabriel, Daniel and the living victims are now finally being treated as human beings instead of just numbers. Which should have happened from the start.
"We sat through eight weeks of the trial and eight weeks of the inquest, every single day and we listened to every piece of evidence.
"We know that Jack should still be here if the officers had done their jobs properly. We live this nightmare every day and we will do for the rest of our lives.
"Whilst we have been told that this may have amounted to gross misconduct, this does not necessarily mean disciplinary actions will take place.
"We hope this is the case and people are held accountable for letting people lose their lives."
IOPC regional director Steve Noonan said: "We recognise it has taken some time to reach this stage, but these are complex matters, involving multiple officers and four investigations into unexplained deaths and then the subsequent murder investigation into Port.
"Though we have found an indication that the behaviour of these eight individuals may have amounted to gross misconduct, this does not necessarily mean disciplinary proceedings will automatically follow.
"Based on the evidence, at the conclusion of our investigation we will decide whether any officers should face disciplinary proceedings."
Cdr Jon Savell from the Met Police reiterated the force's "heartfelt" apologies for its mistakes in the case.
Families of three of Port's victims received compensation from the Met after settling civil claims.
Speaking on behalf of the families of the four men, solicitor Neil Hudgell said they were "cautiously encouraged", adding that the latest development was testament to their "determination and perseverance".