Brian Laundrie's sister urged their parents to talk to the police and 'come clean' if they helped him

• Cassie Laundrie urged her parents to "come clean" if they helped her brother, Brian.
• She told ABC News she didn't know whether her parents had assisted him in any way.
• She said that she last saw Brian on September 6 and that it felt like "just a regular visit."

In an interview broadcast on Tuesday, Brian Laundrie's sister urged her parents to "come clean" if they helped her brother.

Cassie Laundrie told ABC's "Good Morning America" that she didn't know whether her parents had helped her brother in his disappearance but urged them to cooperate with authorities in their search for him.

Brian Laundrie, 23, disappeared five days before the body of his fiancée, Gabby Petito, was found in a Wyoming campsite.

He'd returned from their road trip without her but with her van on September 1. Petito's family reported her missing 10 days later.

Brian Laundrie has been named a person of interest in Petito's disappearance and death, which has been ruled a homicide.

In late September, authorities issued a warrant for his arrest, accusing him of using a debit card without authorization.

His parents said he went missing on September 14. Authorities have searched their home, where he had been presumed to be staying.

"I don't know if my parents are involved," Cassie Laundrie told ABC. "I think if they are, they should come clean."

She added that she was both concerned for and angry with her brother.

"I worry about him. I hope he's OK. And then I'm angry, and I don't know what to think," she told ABC. She urged him to "just come forward and get us out of this horrible mess."

She said she wished he had come to her when he returned with Petito's van.

Cassie Laundrie had said on Monday that she last saw her brother for a few hours on September 6 while he and their parents were on a camping trip.

"There was nothing peculiar about it," she told ABC of the visit. "There was no feeling of grand goodbye. There was no nothing."

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

She also said she had cooperated with the police "since day one."