NSW detectives investigating the case of missing Sydney boy William Tyrrell say they’re frustrated but determined to find him, on the third anniversary of his disappearance.

Detective Chief Inspector Gary Jubelin, who’s leading the investigation, on Tuesday said “hundreds” of persons of interest had been eliminated in the past year.

“I can assure you that we haven’t given up on the investigation, I can assure you we will not give up on the investigation,” he told reporters in Sydney.

“The investigation is currently very active.”

Police are working on the assumption there was “human intervention” in William’s disappearance, although they’re keeping an open mind about whether that involved a pedophile or sex offender.

One theory is that a single person was involved.

Det Chief Insp Jubelin said his team is interested in hearing from anyone who has suspicions about a relative or someone they know, and urged people to take careful note of any “strange” reaction to mentions of William’s case.

“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.”

The three-year-old was last seen playing in his foster grandmother’s yard at Kendall, south of Port Macquarie, on September 12, 2014.

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.

Det Chief Insp Jubelin described that development as a “peripheral distraction” and said it hadn’t harmed the investigation.

William Tyrrell’s foster family has remained out of the media spotlight and Det Chief Inspector Jubelin said they’d endured “a living nightmare”.

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

Police are feeling the pressure to solve the case and end the family’s ongoing grief, Det Insp Jubelin added.

“I can assure you it’s weighing very heavily on all of us.”

A $1 million reward remains on offer for information that leads police to William.

His foster family on Monday spoke of their “unspeakable heartache” and vowed to never give up searching.

“We will never stop looking for you, and until the tomorrow we yearn for comes, we will never give up hope that you will be found and returned home to the arms of your loved ones where you belong,” they wrote in a Facebook post.

A Current Affair publicly revealed William Tyrrell was in foster care after the NSW Court of Appeal ruled against restricting publication of the detail.

The court found it was a matter of “legitimate public interest” to report he was the responsibility of the Department of Family and Community Services when he vanished, according to court documents.

William’s biological parents, Karlie Tyrrell and Brendan Collins, were also named.

– AAP