The lead investigator in the search for William Tyrrell admits he’s frustrated the case remains unsolved and has urged the public to remain vigilant for suspicious behaviour from friends or relatives.

As the missing Sydney boy’s family and foster family marked the third anniversary of his disappearance on Tuesday, Detective Chief Inspector Gary Jubelin said despite the elimination of hundreds of suspects, detectives were still committed to the case.

With the list of persons of interest in the hundreds, police are keeping an open mind about why William might have been taken and police cited research suggesting a three-year-old child may not fit the ideal victim of a pedophile.

“We’re not giving up on this investigation,” he told reporters in Sydney.

“It doesn’t sit well with me personally that three years down the track we haven’t solved (it).”

The seasoned detective pleaded with potential witnesses not to bog police down with theories from clairvoyants or mediums but instead share “genuine information”, including about the behaviour of a friend or relative.

“The way they react when William’s name is mentioned might cause suspicion,” he said.

An unprecedented $1 million reward remains on offer for information leading to William’s return.

Det Chief Insp Jubelin described the crime as once-in-a-generation and said the investigation “weighed heavily” on all those involved.

He described the body of evidence, which includes more than 4000 pieces of information, as “extreme”, but one theory is that a single person abducted William.

“I want that person to feel the pressure,” he said.

“I want that person to feel that everyone’s looking at them and let’s see where that takes us.”

NSW Police Detective Chief Inspector Gary Jubelin briefs urges the public to provide credible evidence. Photo: AAP

William was last seen playing in his foster grandmother’s yard at Kendall, south of Port Macquarie, on September 12, 2014. He would now be six years old.

A Court of Appeal decision last month allowed William’s in-care status to be made public, along with the identity of his biological mother Karlie Tyrrell and father Brendan Collins.

William’s foster family remains out of the media spotlight as they continue to raise his older sister.

“I’ve got nothing but admiration for them, the way they’re handling the situation,” Det Chief Insp Jubelin said.

“It is basically a living nightmare.”

Members of both the biological and foster family have been ruled out as persons of interest.

– AAP