The mother of deceased Gabby Petito says she believes fugitive Brian Laundrie's parents know "most of the information" surrounding her daughter's death "if not everything."

Petito's mother, Nichole Schmidt — as well as her father and stepfather, Joseph Petito and Jim Schmidt — joined "60 Minutes Australia" for an interview published over the weekend to discuss the mystery behind their daughter's disappearance and killing that has captured the world's attention.

"I think silence speaks volumes," Schmidt said of Laundrie's parents in the interview released Sunday. "I believe they know…if not everything, they know most of the information. I would love to just — face to face — ask, ‘Why are you doing this? Just tell me the truth.'"

Petito's parents last spoke to their daughter in late August before reporting her missing on Sept. 11. The 22-year-old was on a cross-country road trip with her fiancé, Brian Laundrie, at the time; the pair were traveling in a Ford Transit van that they had converted into a camper.

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Laundrie, 23, returned home from Wyoming to North Port, Florida, on Sept. 1 in Petito's van without her. Petito lived in Laundrie's home with his parents, Chris and Roberta Laundrie, for a year before they went on their road trip. After Petito was reported missing, Chris and Roberta Laundrie immediately hired an attorney to respond to questions from local police. 

"You'd think there'd be some conversations," Joseph Petito told "60 Minutes Australia." "If Brian was staying at my house and Gabby returned to my house without him in his vehicle, I'd be on the phone with [his] parents saying, ‘Listen, we’ve got an issue. … We've got some stuff to talk about, like, now.'"

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He described the situation as "torturous" and "cruel."

Gabby Petito (Instagram)

The Laundries reported their son missing six days later on Sept. 17, though their attorney later said the last time they spoke with him was Sept. 13. The FBI discovered Petito's remains at a dispersed camping area within the Bridger-Teton National Forest in Moose, Wyoming, on Sept. 19.

Teton County Coroner Brent Blue on Tuesday announced Petito's cause of death as strangulation and said she likely died three to four weeks before her remains were discovered.

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Jim Schmidt described Brian earlier in the interview as "polite," and Nichole Schmidt said he seemed like a "nice guy" whom she trusted to take care of her daughter when she left home with him for their road trip.

"He'd draw them pictures and stuff. He'd read books to my little one at night before bed," Nichole Schmidt said of Laundrie.

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Laundrie is a person of interest in Petito's killing, though his whereabouts remain unknown more than a month after his parents filed a missing persons report. On Sept. 23, the FBI issued an arrest warrant for Laundrie, accusing him of bank card fraud. Authorities alleged at the time that he used an unidentified person’s Capital One card and the personal identification number to charge or withdraw over $1,000 between Aug. 30 and Sept. 1, a period when Petito was missing.

Petito's family created The Gabby Petito Foundation in memory of their daughter to help parents of missing children and victims of domestic violence. The organization held its first fundraiser on Sunday and raised more than $13,700.

Fox News' Michael Ruiz contributed to this report.