Duane 'Dog the Bounty Hunter' Chapman said he "absolutely" believes the the sister of Brian Laundrie -- the fianceé of Gabrielle 'Gabby' Petito and a person of interest in her death -- knows where her brother is.

TMZ shared video footage of Chapman and his wife, Francie, knocking on the door at Cassie's home in Lakewood Ranch, Florida to no answer just before 7:30 p.m. on Wednesday (October 13).

When asked by the person recording if he thinks Cassie is knows more about her brother's whereabouts than she's admitting, Chapman said, "absolutely."

Earlier this month, Cassie Laundrie told ABC News that she didn't know where her was and would "turn him in" if she did, which she reiterated to protesters outside her Florida home in a video shared by TMZ.

Cassie Laundrie said she last heard from her brother on September 6 -- after he returned from his cross country trip without Petito -- when the family went to Fort De Soto Park in Pinellas County, Florida.

"We just went for a couple of hours and we ate dinner and had s'mores around the campfire and left, and there was nothing peculiar about it," she said. "There was no feeling of grand goodbye. There was no nothing."

"I'm frustrated that, in hindsight, I didn't pick up on anything," she added. "It was just a regular visit."

Cassie added that it's unusual for Brian to disappear this long, having initially been reported missing on September 17, days after Petito's parents initially reported her missing.

"I worry about him. I hope he's OK, and then I'm angry and I don't know what to think," she said. "I hope my brother is alive because I want answers just as much as everybody else."

Cassie said she's cooperated with authorities during their investigation "since day one" and believes her parents should do the same, noting that she's unsure of their role.

"I don't know if my parents are involved," she said. "I think if they are, then they should come clean."

Laundrie's parents claim the 23-year-old was last seen hiking in Carlton Reserve on September 13 after he returned to Florida from a cross country trip without Petito.

On Monday (October 11), Chapman the Sun that he not only thinks Laundrie -- who has not been charged in the case -- killed Petito, but also called his parents at the scene of her death, who he believes advised him to return to their home in Florida.

"Of course he murdered her. I think he called the mom and dad right from the scene and said 'omg.' Now we have to remember that they lived -- Gabby and Brian -- with his mom and dad for almost two years," Chapman said while piecing together how he believes an alleged phone call between Brian Laundrie and his parents, Chris and Roberta Laundrie, sounded. "And the house, I was there, it’s not a huge house so I’m sure they heard, the parents probably heard Gabby screaming. I was told by a very close source that he was a gentleman.

"I think he said, 'mom, she was screaming, she was screaming so I put my hand over her mouth, and I held my hand and she was screaming and when I took it away she wasn’t breathing and I tried CPR'."

On Tuesday (October 12), Teton County Coroner Dr. Brent Blue -- who performed an autopsy on Petito -- announced her death was ruled as a death by strangulation and the manner of death to be a homicide during a press conference on Tuesday (October 12).

Dr. Blue said Petito's death is believed to have occurred 3-4 weeks prior to her remains being found on September 19 and confirmed to match her days later.

Dr. Blue confirmed the ruling was made while working alongside local and federal authorities. No other information will be released in adherence with state law.

Laundrie is currently at the center of a publicized search by the Federal Bureau of Investigation and local authorities at the Carlton Reserve campsite in Venice, Florida.

Dr. Blue said local law enforcement would decide who would be charged in connection to Petito's homicide case.

On October 7, Chris Laundrie joined authorities in their search of Carlton Reserve for his son, according to family attorney Steven Bertolino.

Remnants of a campsite that appeared to have been recently used were reportedly the focus of the search, a source close to the Laundrie family told CNN's Chris Cuomo last Wednesday (October 6).

On September 30, police released additional bodycam footage which shows Petito telling an officer that an argument with Laundrie got physical.

The newer footage stems from the incident on August 12 in Moab, Utah in which police made contact with Petito and Laundrie after a bystander called the Moab Police Department and reported seeing a man hitting a woman.