They were either visiting, residing or working in the small NSW town of Kendall that day.

They were acting on impulse, they took tremendous risks and they almost certainly made mistakes.

That is the chilling profile that police are building of the person or persons who snatched three-year-old William Tyrrell from the front yard of his grandmother's home last year.

As William's family gathered with residents of Kendall on Saturday to mark one year since the toddler's disappearance, the senior criminal analyst working with Strike Force Rosann, Sarah Yule, provided The Sun-Herald with an insight into the mind of William's abductor.

The "little Spider-Man" should have been spending Saturday like any other boy his age. He would most likely have been "[playing] sport, playing with friends and riding bikes as a family", his family said.

Instead, investigators are tirelessly working to find the person who snatched him 12 months earlier, at 10.30am on a Friday, while his mother was inside making a cup of tea and his sister and grandmother were sitting on the back porch.

Forensic psychologist Dr Yule said police were working on a reasonably narrow abduction scenario.

"There may be a misconception that there is a lot that we don't know about this case. I've visited the crime scene multiple times and there is a lot of information that that location tells us," Dr Yule said.

"Combining that with assessing all the leads that have come into the investigation so far, there really are a limited number of possibilities that can account for William's disappearance and from that there are actually a lot of inferences we can draw about what has likely happened and the likely personality of a person or persons who may be involved."

William vanished from a dead-end street on the edge of a small town. No one but his parents and grandmother knew in advance of his visit to Kendall.

This has made police confident that the person who kidnapped him was already in the area.

"You would have to have some other reason to be there and take that opportunity; either visiting, residing or working in the vicinity."

There was only a narrow window of opportunity to take William, who was snatched in broad daylight and with a significant risk of being seen by someone on the balcony of William's home as well as neighbouring properties.

This has made police certain that the kidnapper was impulsive and took tremendous risks.

And if they took tremendous risks, mistakes were almost certainly made.

"It does seem that this was an opportunistic crime and when somebody makes that kind of impulsive decision, mistakes are made and it's those kind of mistakes that the investigation is focusing on," Dr Yule said.

No one has been arrested over William's disappearance and one person of interest has been publicly identified, washing machine repairman Bill Spedding.

He released a YouTube video last week expressing his "sincere commiserations" to the Tyrrell family and strenuously denying any involvement.

William's family said they would be spending Saturday "being a family, remembering our gorgeous little boy [and] hoping and praying that someone finally realises it's time to come forward with information".

They said the past 12 months has been "a year we wish never happened".

"We've had to rebuild our family and do things we never in our wildest dreams thought we would need to," his parents said.

They once again renewed their plea for anyone with information to come forward.

"To the person who knows something about what happened to William, it's been too long. We don't understand how you can keep quiet. How much longer can you carry this burden?

"Please, please come forward and tell police what you know so they can find William and bring him home."