The father of missing Iowa college student Mollie Tibbetts thinks she is alive and was taken by someone she knows.

“I just don’t think that anyone set out to harm Mollie,” Rob Tibbetts told Fox News on Monday. “And the longer we go without finding her, the more optimistic we are that she is with someone that doesn’t want to harm her but doesn’t know how to extricate themselves from this situation."

"My belief is that she left willingly with someone she knows under whatever pretense and whatever circumstance that changed," he said.

The search for Tibbetts is in its third week as investigators continue to follow leads and interview residents in the area.

One resident, a farmer in Deep River, Iowa, 13 miles southeast of Brooklyn where Tibbetts was last seen, has been questioned multiple times, according to the Des Moines Register. On Monday, the farmer, Wayne Cheney, 54, refused to take a polygraph, the paper reported.

"I don't need to," Cheney said, citing the fact that he had previously spoken with investigators for over an hour and allowed them to search his cellphone, house and 70-acre farm. "It's stupid."

Cheney has not been named a suspect, the Des Moines Register reported.

As of Monday, the reward offered by authorities for information on Tibbetts’ whereabouts was more than $270,000, according to the Crime Stoppers of Central Iowa Facebook page.

Tibbetts was last seen on July 18 during an evening jog in Brooklyn. She was wearing dark running shorts, a pink sports top and running shoes.

Tibbetts' father said he is still hoping for her safe return.

"This is just a hope more than a theory," Rob Tibbetts told Fox News. "But, the longer we go without finding Mollie, the more I believe she is somewhere where we can get her back."