The anonymous tip that cracked open the case of Tiahleigh Palmer's murder.

On Friday, the man who was meant to protect 12-year-old Tiahleigh Palmer – her foster father Rick Thorburn – was sentenced to life in jail for her murder.

The Queensland girl had been placed in his family’s care in January 2015, but 57-year-old Thorburn killed her nine months later and dumped her body beside a Gold Coast river.

The calculated betrayal was an attempt to cover up his 18-year-old son Trent’s incestuous abuse of the girl, fearing she was pregnant, and keep him out of jail.

New details have since emerged outlining how police came to reconsider the Thorburns as suspects, after they’d been playing the role of the grieving family.

Channel Nine current affairs program 60 Minutes reports that it was a phone call in May 2016 that put detectives’ focus back on the Thorburns, who’d claimed Tiahleigh vanished after leaving for school.

“An anonymous call came through,” Detective Damien Hansen told reporter Tara Brown.

“There was a Facebook message sent from Trent outlining a sexual activity with Tiahleigh, and that there’d been a family meeting the night before she disappeared.”

The Queensland Police Homicide investigator said the message from Trent was revealing to his cousin he had sex with his foster sister and was worried she was pregnant.

He also reportedly wrote he wanted the girl “out of his life” despite her being “a source of income” for his parents.

“You look at the content of that Facebook message, you look at the fact there’s a family meeting and just by coincidence she goes missing the next day… It’s suspicious,” Detective Hansen said.

Upon realising the killer was hiding in plain sight all along, police bugged the Thorburn home and captured Rick and wife Julene discussing the deception and coaching their sons.

Armed with mounting evidence, police managed to get confessions from the family.

Josh told police: “(My dad) didn’t cry. He seemed quite assured and defensive of what he’d done and why he’d done it.”

Thorburn will be eligible for parole in September 2036.

Julene, Josh and Trent have also all served time in jail for their role in Tiahleigh’s death, but they have since been released.

On Friday, Tiahleigh’s biological mother, Cindy, said she was devastated that Thorburn killed a beautiful girl with her life ahead of her.

“Rick Thorburn took that away from me, from our family and most of all Tiahleigh,” she said outside court.

– With AAP