August 09, 2018 03:54 PM

As the search for answers continues in the baffling disappearance of Iowa college student Mollie Tibbetts, her boyfriend’s brother says this is certain: If she was abducted, she didn’t go willingly.

“She’s small, but she would have done something,” said Blake Jack, speaking to Fox News Wednesday.

Jack owns the Brooklyn, Iowa, home Tibbetts and her boyfriend, Dalton Jack, were living in this summer. Tibbetts, 20, was last seen alive on July 18, following her nightly run.

The Jack brothers were both out of town, working on construction sites, at the time of her disappearance. Tibbetts had been dog-sitting for Blake.

A reward of more than $312,000 has been raised for anyone who can provide critical information ensuring Tibbetts’ safe return.

WATCH: 20-Year-Old College Student in Iowa Vanishes on Jog

Blake, 23, told Fox News he has no idea if Tibbetts returned home from her run that July night (she was reported missing the next day), but he noted there were no signs of struggle at the house.

“If something happened while on a run, people — like you see right now — our neighbors are outside and would have heard something,” he said.

Speculation that Tibbetts could not have been abducted by a stranger is misguided, Mitch Mortvedt, a spokesman for the Iowa Division of Criminal Investigation, previously told PEOPLE.

“One avenue of thought is that if it wasn’t a struggle, and we have found no signs of a struggle, then maybe it was someone she was familiar with,” he said. “But then the other avenue of thought is it could have been a stranger who simply overpowered her or tricked her. We just have very little to go on.”

RELATED: Mystery Without End — Everything We Know About Missing College Student Mollie Tibbetts

From left: Dalton Jack and Mollie Tibbetts
Courtesy Emily Heaston

Tibbetts, who was studying psychology at the University of Iowa, has not been seen since going running more than three weeks ago. She was reported missing after failing to show up for her job as a camp counselor.

Her phone and fitness tracker, which she apparently took with her while jogging, have also not been recovered. However, her family is steadfast in their belief she is alive — but in danger.

“It’s totally speculation, but I do believe that Mollie is with someone who she knows, probably someone who cares about her,” dad Rob Tibbetts has said. “But that relationship was misguided, misinterpreted and went wrong, and I think they’re in a place with Mollie and don’t know how to get themselves out of this horrible situation.”

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Addressing her possible abductor, Mollie’s father said on ABC News: “You’ve made a mistake. We’ve all made mistakes. Don’t compound this. Work your way through this. Listen to Mollie.”

Mollie is described as 5-foot-3-inches tall, weighing about 120 lbs., with long brown hair and brown eyes.

Anyone with information about her is urged to contact authorities at 800-452-1111 or 515-223-1400 or email

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