For months, Rick Thorburn insisted he had nothing to do with his foster daughter's death. Then police discovered the statements from his family that pointed to a horrible crime.

As BBC News reports, 12-year-old Tiahleigh Palmer had been in Australia's foster care system for about five years before she was placed with the Thorburn family in January 2015.

Screenshot/9News

Tiahleigh's mother, Cindy Palmer, told abc.net.au that she'd put her children in foster care in order to protect them from a “severe domestic violence” situation:

“It's not what I wanted, and it wasn't the outcome any of us wanted, but at the end of the day I thought that was the safer option.”

Battling addiction and homelessness, Cindy wasn't able to take her daughter back. But when she heard that Tiahleigh had been placed with a family who had teenage boys, she was upset. Her misgivings got worse when she saw how her daughter reacted to her new home.

She told abc.net.au that Tiahleigh began wearing more mature clothing (like padded bras), and was running away regularly. It seemed to the concerned mom that her daughter didn't want to be left with Rick Thorburn, her new foster father.

On October 30, 2015, Rick Thorburn reported Tiahleigh missing. He told authorities he had last seen her when he dropped her at school that morning. He helped police in the search and seemed extremely emotional over her disappearance.

Six days later, Tiahleigh's body was found, nearly naked, on a riverbank about 30 miles from the Thorburn home. While police investigated Tiahleigh's death, Rick was one of her pallbearers at a funeral attended by 600 mourners.

For months, police struggled to find leads that would solve Tiahleigh's murder. Then they got a tip about a Facebook message that Trent Thorburn, the younger son of the family, had sent to a cousin. According to abc.net.au, the teenager had confessed to having sex with his 12-year-old foster sister:

I'm in trouble and I don't know what to do. I have never had sex before and she [Tiahleigh] made me do that with her.

I just want the kid gone and out of my life but I know she is also a source of income for mum and dad as well.

Police bugged the Thorburn home and were able to record evidence of a family cover-up regarding Tiahleigh's death. Mom Julene reportedly said, “We have to stick to the same story about her going to school the next day and whatever.” Meanwhile, Rick coached Trent on a story about Tiahleigh coming into his room while he was drunk.

The police recordings also made it clear that Julene and older son Josh were beginning to weary of the lies and secrecy. In one recording, Julene talks about coming clean in order to protect herself and her sons, making it clear that she put the responsibility for Tiahleigh's death on Rick.

Authorities were able to use the recordings to persuade Julene and Josh to confess. The story that came out of their testimony was horrifying.

When Rick claimed to have taken Tiahleigh to school on October 30, he was lying. He had actually killed her the night before. When he made a missing persons report, her body was still on his property. While the search went on, he disposed of her body on the riverbank.

Police learned that after discovering Trent had sexually assaulted Tiahleigh, Rick was afraid the young girl was pregnant and that his son would go to prison. He killed Tiahleigh in order to cover up his son's crime.

On October 29, Rick told Julene that they had to think carefully about how to handle their son's assault of their foster daughter. She left him alone with Tiahleigh that night. The Courier Mail reports that when Julene came home and asked whether the girl was okay, he told her:

“It’s all taken care of ... Don’t ask any questions you don’t need to know I have taken care of it.”

According to abc.net.au, Josh Thorburn told police the family had made a pact of secrecy, and that it was obvious what had happened to Tiahleigh:

“His exact words were: 'Tiah is no longer with us.' And also he said, 'I hope you understand what that means.' I knew what he meant — that he'd killed her.”

He added:

“He didn't cry — he seemed quite assured and defensive of what he'd done and why he had done it — obviously it was to protect Trent.”

Josh served three months in jail for lying to the police, and Julene served six months on the same charge. Trent was convicted of incest and perjury. Though sentenced to four years, he was released on parole in May.

Josh told police his father didn't explain how he had killed Tiahleigh, instead saying, “that it's best that we don't know how it happened, because the less we know, the less we can tell.”

Charged with Tiahleigh's murder, Rick pleaded guilty and was sentenced to 20 years in prison. According to the Courier Mail, in handing down the sentence, Judge David Boddice said:

“You murdered this defenseless child, who relied on you for protection ... and you did so to save one of your own children because of the consequences of his actions.” 

He added: “You showed no respect for her even in death.”