Detectives searching for William Tyrrell have shifted their attention to a concrete floor in the garage of his former foster grandmother’s home.

Officers were also digging again in areas around the Kendall home on Thursday, a day after police revealed they had seized a car that once belonged to the little boy’s foster grandmother.

On Thursday, Australian Federal Police officers were pictured using ground-penetrating radar on the concrete slab in the woman’s former home, on the NSW mid-north coast.

The slab is said to have been laid since three-year-old William disappeared in September 2014.

It comes after former lead detective in charge of the case revealed he bugged the car of the missing boy’s foster-parents before they were ruled out suspects.

In an interview with 2GB radio on Thursday, former homicide detective Gary Jubelin said he had formally interrogated and released William’s foster parents, while also running a “covert operation” that involved listening in on their conversations.

“Based on the answers to those questions and also the information that we gathered during the covert operation, they were again eliminated,” Mr Jubelin said.

He led the William Tyrrell investigation in 2015 until he was removed from the case in March 2019. He resigned from the police force shortly after.

During the interview, Mr Jubelin said he took over the case five months after William vanished. He was told then that the foster parents had been ruled out of involvement.

William Tyrrell vanished from his foster grandmother’s house in 2014. Photo: AAP

Mr Jubelin said the theory William possibly died after falling from a balcony at the house in Kendall was investigated back in 2016. Police have spent hours this week digging up a garden underneath the balcony.

He also ruled out questions around the foster mother’s account that William was wearing shoes when he was last seen.

William was barefoot when he went missing.

“[We] investigated every theory: [He] died from an accident, whether William was run over on the driveway or fell over and hit his head on a rock or fell off the balcony,” Mr Jubelin said.

“That’s the most obvious place, when you look at the house, that a child could injure themselves.”

Mr Jubelin was convicted of four illegal recordings of interviews with a person of interest in April 2020. He was fined $10,000.

Police Commissioner Mick Fuller this week said the new investigation team had “inherited what was a bit of a mess”.

Mr Jubelin took issue with this, saying he provided monthly progress reports to his superior officers detailing everything – “what suspects I was targeting, what the future directions were”.

Mr Jubelin admitted he had become friends with William’s foster parents and considered the foster mother “a very decent human being”.

He said he went hard when investigating the couple.

“I basically ambushed the [foster] parents and then I interrogated the [foster] parents,” he said.

On Wednesday, William’s former foster parents were charged over the alleged assault of a different child.

– with AAP