The NSW Police Homicide Squad member said he thought it was 'highly likely' police had already come across the perpetrator of the crime.
'I would be sweating.' Jubelin said.
William, three at the time of his disappearance, was taken three years ago outside of his home in Kendall, New South Wales.
From thousands of persons of interest, police have managed to narrow their pool to a few hundred. The police also now believe the perpetrator was acting alone.
Previously, police had investigated an alleged pedophile ring operating around Kendall as possibly being involved in William's disappearance, but a recent report by A Current Affair has revealed that line of inquiry can be ruled out.
"Even if it is one person involved, I am sure that there's people close to that person that have suspicions about that person," Det. Jubelin said.
The NSW government has announced an unprecedented $1 million reward for information leading to William's whereabouts.
This article originally appeared on New Idea.