Updated

Blake Jack, the brother of Mollie Tibbetts’ boyfriend, told Fox News exclusively on Wednesday that there was no sign of struggle at the boyfriend's Brooklyn, Iowa, home one day after she vanished.

“She’s small, but she would have done something,” he said.

Tibbetts, 20, went missing July 18 from the town, which is about 70 miles east of Des Moines. She was last seen jogging at about 7:30 p.m. in a place where one community watch organizer said “not a lot of big things happen.”

Jack and his brother said Mollie’s cell phone, Fitbit, ear buds and arm band - which she used while running - are missing from the home. They also said Mollie rarely stayed overnight at their home alone. 

Officers from the FBI, state and local law enforcement agencies have been working to find her. The reward fund for the missing University of Iowa sophomore has grown to more than $300,000 and is likely to continue climbing, a spokesman for Crime Stoppers of Central Iowa said.

Tibbetts' boyfriend, Dalton Jack, has said he opened a Snapchat message from her at around 10:15 p.m. on the night of July 18, but it’s unclear when, exactly, the Snapchat was sent.

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

— Blake Jack, the brother of Mollie Tibbetts’ boyfriend

“We just don’t know,” Jack said of the Snapchat, which appeared to have been taken indoors.

Tibbetts was staying at her boyfriend's home at the time of her disappearance. She was there to watch his dogs because he had a construction job about 100 miles northeast in Dubuque.

The morning after she disappeared, Tibbetts' family reported her missing after she didn't show up to her job at a daycare center in a nearby town.

Blake Jack, 23, told Fox News that it is impossible to know if she ever returned to the home after her run. Investigators and scent dogs were at the property “a bunch of times.”

Mollie Tibbetts

Drone footage captures the last known location of Mollie Tibbetts in Iowa.

He described the town of 1,400 as a place where residents don’t lock their doors.

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

Allie Houghton said she believes Tibbetts went somewhere with someone she knew.

“There was no kicking of anything. There was no obvious sign of struggle,” she said.

Bill Moulder, Des Moines' longtime police chief, told the Des Moines Register that missing persons cases are usually among the most difficult to solve in law enforcement. He said cases are even more challenging when the suspect is a stranger.

"There's no connection between the person and the stranger. In personal crimes, the subject is connected to the victim — spouse, friend, competitor, someone they fought with. You have a starting point," he said. "But with a stranger abduction, there's no connection.”

The Associated Press contributed to this report.