The boyfriend of missing Iowa college student Mollie Tibbetts said each day since her disappearance has been harder than the one before.

“Initially, it was shock and then, you know, it gets harder day by day because she's still not here and you miss her, you miss her more than anything in the world,” Dalton Jack said in an interview.

Jack, who was working a job in Dubuque, Iowa, on the night Tibbetts went missing, is not a suspect in the investigation.

Tibbetts, 20, was last seen jogging on the evening of July 18 in Brooklyn, Iowa, said Kevin Winker of the Iowa Department of Public Safety. Tibbetts was believed to be wearing dark running shorts, a pink sports top and running shoes, according to police.

She was reported missing the following day after not showing up to her job at a day care.

Jack told the TV station KCRG that Tibbetts had sent him a Snapchat late on July 18, presumably after she had gotten back from her run.

“She's not going to run off," Jack said in an interview. “I try not to speculate on it too much because the only thing that comes into your head whenever you're not investigating all the facts is that something bad happened and you don't, I personally don't want to believe that."

Tibbetts' parents expressed their anguish to reporters on Thursday.

"Every day I feel Mollie's presence with me," Tibbetts' mother said. "Sometimes I feel her sitting on my shoulder. Mollie was an incredibly strong young woman. I don't know that I have the strength but Mollie is lending me her strength"

"Everyone in the media, community, and country has adopted Mollie so let's keep at it," Tibbetts' father said. "Let's go get her."

Tibbetts' parents also announced a new $172,000 reward for anyone that contacts crime stoppers of central Iowa with information.

Investigators are looking to explore Tibbetts’ digital footprint, specifically looking at a Fitbit device which she would have been wearing during her jog, according to NBC affiliate KWWL.