Mollie Tibbetts search: Police investigating 'hundreds' of leads

A reward to find missing US university student Mollie Tibbetts has increased to $350,000 AUD.

The 20-year-old vanished from a small town in Iowa during an evening jog on July 18.

RELATED: FBI joins hunt for missing student Mollie Tibbetts in Iowa

Her family is offering the money, believing their daughter may have been kidnapped.

"We believe that Mollie is still alive, and if someone has abducted her, we are pleading with you to please release her," Mollie's mother, Laura Calderwood, told a media conference.

RELATED: Family of missing US student crowdfund for her return: ‘She’s out there’

Investigators say the case remains one of their most high-profile, with resources pulled in from other departments.

"We’ve been following up on leads," said Kevin Winker, Director of Investigative Operations at the Iowa Department of Public Safety, according to Fox News.

"Those leads are in the hundreds. Investigators from the Poweshiek County Sheriff’s Office, the FBI, the Division of Criminal Investigation.

"There are an average 30 to 40 working this case a day."

Officers from the FBI returned to a pig farm to question the owner in connection to Mollie's disappearance. It was the second time Wayne Cheney was spoken to by investigators and he continues to deny any involvement, reported Fox News.

He told the media organisation the FBI had has searched his home, some of his property and his mobile phone.

He's refused to take a polygraph test despite a request by law enforcement.

Search teams have also again scoured ditches close to the farm.

On Sunday, there were hopes of a breakthrough when the body of a woman in her 20s was found an hour from where Mollie lived, in Brooklyn.

But investigators have confirmed the body does not belong to the missing University of Iowa student.

Mollie's boyfriend, Dalton Jack, has been ruled out as a suspect because he was working at a construction site 160 kilometres away at the time of her disappearance.

RELATED: Boyfriend of missing student Mollie Tibbetts speaks out about shock disappearance

She was staying at his home to look after his dogs, when she decided to go for a run on the evening of her disappearance.

She has not been seen since.