Serial killer Levi Bellfield was part of a child sex gang that has never been brought to justice, a report claims.
The 50-year-old was jailed for life in 2011 for murdering 13-year-old Milly Dowler and two women.
The report by senior social worker Debbie Weissang sent to the Met Police linked Bellfield with six men accused of paedophilia, murder and grooming.
Ms Weissang said she was "deeply concerned there remained a risk to children" from Bellfield's associates.
Bellfield was handed a whole life sentence after confessing to Milly's murder.
He had already been given life sentences in 2008 for the murders of Amélie Delagrange, a 22-year-old French student, and 19-year-old Marsha McDonnell.
Ms Weissang, the former child sexual exploitation manager at Hillingdon Council in West London, wrote her report on Bellfield, who had lived in the authority's area, in December 2017.
Ms Weissang said that despite Bellfield being "locked away" there were "six other men" who were not serving life sentences and "pose a serious threat to children".
She added she wanted the Dowler family to know there were "many people that are working really hard to make a difference for the survivors".
Other alleged gang members include Victor Kelly and Suraj Gharu, who have both been convicted of child sex offences, and a third man who was found guilty of murder.
However, Ms Weissang believes the other three members of the gang remain in the community, though they may have been arrested since the report.
A Met Police Spokesman said: "A safeguarding report was received from Hillingdon Council.
"Any allegations of Child Sexual Exploitation contained in the report will be assessed and be investigated where appropriate."
A spokesman for Hillingdon Council added: "Following completion of the report the information was carefully reviewed by Hillingdon Council's Head of Safeguarding in partnership with the Metropolitan Police to ensure a timely and proportionate response.
"A dedicated police resource was also assigned to review the information. Hillingdon Council's Children's Social Care team conducted numerous safeguarding checks including liaison with all relevant agencies."