Audio released after murdered teen hikers "recorded their killer's voice" before deaths.

Indiana State Police have released a chilling audio recording of the voice of the prime suspect who is accused of killing two teenage girls in Indiana last week.

Liberty German, 14, and 13-year-old Abigail Williams, went missing after they were dropped off by a family member for a hike at Monon High Bridge east of Delphi on Monday last week.

Just one hour after authorities reported them missing in the media, their bodies were found in nearby Deer Creek.

Authorities revealed that their deaths were being treated as a "double homicide", but have not yet confirmed how they died.

The pair posted a series of images to Snapchat before their deaths, and now police have revealed the girls also recorded a video of their suspected killer.

While Indiana State Police are not releasing the video, they did play an audio clip of a man's voice during a news conference.

The man can be heard ordering the girls "down the hill".

Captain David Bursten said the audio came from 14-year-old Liberty German's phone. He also confirmed the grainy image of a suspect released by police last week also came from her phone.

"She had the presence of mind to have the phone on and to capture video as well as audio," Bursten said.

It is unclear whether the voice and the image belong to the same man. He is yet to be identified and no arrests have been made in relation to the case.

Police are appealing for help from the public to identify the man's voice.

Listen: What it's like when a loved one goes missing. Loren O'Keeffe shares her story.