Operation Equinox was launched in 2011 into allegations of attacks in Nottinghamshire children's homes since the 1950s.
But the inquiry has faced criticism that, despite recording about 400 potential abusers, only three people have been jailed.
Officials said a lot of evidence had been lost over the decades.
Equinox has seen more than 300 men and women make 680 separate allegations.
Head of the operation, Det Supt Rob Griffin, said the figure of 400 suspects had to qualified: "Many of those are unidentified and actually all of the unidentified suspects could be one and the same person and we know that over 50 of them are dead.
"These days we rely so heavily of forensic evidence, phone evidence, CCTV, things like that.
"But when we are dealing with historical allegations and historical investigations such as Operation Equinox we don't have that benefit, so it makes it all the more challenging."
The authorities said sometimes this lack of evidence meant there was no realistic prospect of a conviction, and prosecutions were not brought.
However survivors were assured those who came forward would still be supported and believed.
But David Hollas, from the Nottinghamshire Child Sexual Abuse Inquiry Action Group, said the situation was unacceptable.
"We are very concerned with the number of cases going to court because there is absolutely no deterrent to people who abuse.
"And there is no incentive there to people who have been abused, including rape victims to come forward.
"More cases should be allowed to go to court and if the guy is found not guilty then so be it, he is found not guilty, at least the system has been seen to be done."
More than 50 suspects in a historical child abuse investigation have already died, police have revealed.