Better care 'may not have prevented double murder'.
The report, chaired by Robert Francis, QC, found there were "significant failings" in the services that dealt with Stone but the authors were "unable to say that the murders could have been avoided by a better standard of care".
Shaun Russell said: "It boils down to if everybody had done their jobs right then there's still no telling, Michael Stone might still have done what he did.
"I think you can just change one word of that: if everybody had done their job right, perhaps he wouldn't have done what he did."
The report found Stone - who had a history of mental disorder, drug abuse and violence - warned a psychiatric nurse he would kill five days before he attacked Lin and Megan and Megan's sister, Josie, then nine, as they walked home from a swimming gala.
He also told doctors that he felt like killing children on more than one occasion.
The "principal and overriding criticism" of the report, commissioned by West Kent Health Authority, Kent Social Services and Kent Probation Service, was that there was a "frequent failure" to apply principles of the Care Programme Approach in sharing information and co-ordinating risk assessment and management programmes.
The report said there was no suggestion that Stone "was deprived of any service which would have made him less of a danger to the public".
"This is emphatically not a case of a man with a dangerous personality disorder being generally ignored by agencies or left at large without supervision."
Stone is serving three life sentences for the murders of Lin and Megan Russell, and the attempted murder of Josie.
Dr Russell said: "Everybody told us he did it, but there's always that tiny niggling doubt because he himself won't admit it."
Stone's sister, Barbara Stone, said: "The reason that he won't admit it, he'll never admit it, is quite simply, he didn't do it."