Ex-NFL Star Joins the Hunt for Indiana Girls' Killer: 'Let's Rid the World of This Monster'

Pat McAfee has doubled the reward to find the murderer and has recorded a PSA with police.

A former NFL superstar is joining the hunt for the killer of two teenage girls in Indiana.

Read: Speculation Grows as Hunt Intensifies for Killer of 2 Teen Hikers in Indiana

Now, Pat McAfee, a popular Indianapolis Colts punter who just retired, has doubled the reward money to more than $200,000 and made a public service announcement with Indiana State Police.

Abigail Williams, 13, and her best friend Liberty German, 14, were murdered on a remote hiking trail on February 13.

The only clues to emerge from the case are a grainy photo of the suspect taken by Liberty on her cellphone and an eerie recording of the killer's voice.

Radio stations across the Midwest are also playing an appeal from cops with the haunting speech from the killer saying, 'Down the hill,’ asking if anyone recognizes the voice.

"If somebody knows who this is, let's rid the world of this monster," McAfee told Inside Edition.

He said he believes someone is agonizing over whether to turn in the suspected killer because he may be a friend or a family member.

Read: Citizen Sleuths Spring Into Action in Indiana Murder Mystery as Reward Reaches $50G

"Anybody that's going to recognize this guy from this picture and his voice is going to be somebody that has known him for a long time, a long, long time so they're trying to balance a loyalty to somebody," the former punter said. "But this is a horrible human being. The world is much better without bad people in it."

Watch: Police Name Mystery Man Caught on Surveillance as Suspect in Murder of Teen Hikers