Their findings go against a coroner’s ruling that it was likely that Williams was unlawfully killed.
Williams’ family is standing by the coroner's findings.
"We are naturally disappointed that it is still not possible to state with certainty how Gareth died and the fact that the circumstances of his death are still unknown adds to our grief,” his family said in a statement. "We consider that on the basis of the facts at present known, the coroner's verdict accurately reflects the circumstances of Gareth's death.”
Deputy Assistant Commissioner Martin Hewitt said that "many questions remain unanswered" as to the circumstances of Williams’ death, but said there was no evidence that the spy’s death was connected to his work.
"I do not believe that I have had the wool pulled over my eyes. I believe that what we are dealing with is a tragic unexplained death," Hewitt said. "Now at the end of our investigation, based on the evidence, or where we have been unable to find positive evidence, we believe that it is a more probable conclusion that there was no other person present when Gareth died.”
He did address the issue of the police coming to a different conclusion than the coroner.
"But the reality is that for both hypotheses, there exist evidential contradictions and gaps in our understanding."
Celebrity Families of 2014 [PHOTOS]