Authorities 'hopeful' missing University of Iowa student is 'still alive' as search enters 10th day

It's been 10 days since 20-year-old Mollie Tibbetts mysteriously vanished in a rural Iowa town, but investigators say they haven't given up hope that they will find her alive.

"At this point, we don't know her exact whereabouts but we continue to look. We are hopeful that she is still alive and so we will continue to think that way until told otherwise," Richard Rahn, special agent in charge of the Iowa Department of Public Safety's Division of Criminal Investigation, told ABC News on Friday night.

"We're doing everything we can to locate her and track her down," he added.

Tibbetts, a rising sophomore at the University of Iowa, went for a jog the evening of July 18 in Brooklyn, Iowa, a close-knit farming community of about 1,500 people where she is staying with her boyfriend for the summer. She never returned.

She was reported missing the following day when she didn't turn up for work.

"That is so out of Mollie's character, one, not to show up for work, two, not to let her employer know, and, three, boyfriend, myself and her brother -- just no one was notified that she didn't go to work," Tibbetts' mother, Laura Calderwood, told ABC News in an exclusive interview Friday.

Tibbetts' boyfriend, 20-year-old Dalton Jack, told ABC News he last saw his girlfriend July 16. The next day, he traveled to the city of Dubuque, Iowa, for his work at a construction company.

He said she stayed at his Brooklyn house alone and watched his dogs. Going for an evening run, he added, was her regular routine.

"She goes for a run every night. She likes to go whenever the sun's not down, but it’s starting to cool off, like 6 or 7," Jack told ABC News in an interview Monday. "She knows Brooklyn extremely well, better than I do, and I’ve lived here my entire life."

Jack said he believes Tibbetts would have had both her fitness tracker and her cell phone when she went out for her jog.

"She probably has on her FitBit because she never takes it off. She uses it for the sleep tracker and for all her runs and everything," he said. "She probably has her cell phone, but it’s either off or dead and we’ve tried calling it obviously and it goes straight to voicemail."

Data from Tibbetts' FitBit, a GPS-enabled activity tracker, has been "helpful" and could be "key" in locating her, according to Rahn.

"It certainly helps in that we are able to help establish a timeline and that's certainly critical for any investigation like this," he added.

Rahn declined to say whether investigators have found her FitBit or if they know its location.

Authorities, with the help of hundreds of volunteers, have scoured swaths of land in Brooklyn and the surrounding Poweshiek County while looking for Tibbetts. They recently searched a pig farm several times near Guernsey, Iowa, about 10 miles south of Brooklyn.

Rahn told ABC News it's "not uncommon" to go back to a site and conduct additional searches as tips come in. He declined to reveal what clues if any investigators may have found at the pig farm. But he said one thing is for certain -- Tibbetts isn't there.

"I can tell you she's not there. Aside from that, I'm not sure what I'm able to release, but she's certainly not there," Rahn said.

Investigators have also canvassed neighborhoods and conducted dozens of interviews. Thus far, no one has been identified as a suspect nor has anyone been arrested or criminally charged for Tibbetts' disappearance, according to the Iowa Department of Public Safety's Division of Criminal Investigation.

As each day passes, Tibbetts' mother, Calderwood, said the uncertainty has been "excruciating."

The mother, who also lives in Brooklyn, said her daughter was looking forward to new, exciting activities in her life. Tibbetts, who is the middle child in her family, was expected to move into her first apartment and start her sophomore year at the University of Iowa this fall. She was also planning a trip to the Dominican Republic with her boyfriend next month to attend his brother's wedding, according to Calderwood.

Still, she said, their family remains hopeful.

"If it were me that were missing, Mollie wouldn't give up hope -- that's not even a thought," Calderwood told ABC News. "You won't see me giving up hope. That's not an option."

Tibbetts was last seen wearing dark-colored running shorts, a pink sports top and running shoes. Anyone with information related to her disappearance can call the Poweshiek County Sheriff's Office at 641-623-5679 or send an email to tips@poweshieckcosheriff.com.

"I've been telling a lot of people that they often see things that they don't believe is critical when in fact it can be," Rahn said. "Even if you don't think it's important, call us so we can then decide and run out that lead."

ABC News' Andy Fies, Rob Hess, Amanda McMaster, Alex Perez and Emily Shapiro contributed to this report.