A voice recording retrieved from the phone of one of two teenage girls killed in Indiana has captured audio of a male voice saying "down the hill".

Liberty German, 14, and Abigail Williams, 13, were killed after going on a hike along a trail near their hometown of Delphi.

Their bodies were found close to a disused railway bridge by the trail. Police in Indiana have released an image of a man seen walking by the trail, who they believe is the main suspect in the case.

"Evidence in this case has led investigators to believe that this is a double homicide, and that's what we're investigating at this time," Indiana State Police spokesman Sgt. Tony Slocum, adding of the man spotted walking by the trail where the girls went missing:

"We are actively looking for this person. We believe this person is our suspect."

Now, audio recorded from the phone of Liberty German, known as Libby, has provided police with a further clue.

"Libby had the presence of mind to turn on her video camera," Indiana State Police spokesman Capt. David Bursten said in comments carried by ABC News.

"There's no doubt in our minds that that young lady is a hero."

A deep voice can be heard on the audio saying "down the hill", police said. Police added there could be more than one suspect and that the audio recorded on Liberty's phone was not necessarily the voice of the man in the photograph.

The funerals of the teenagers, who enjoyed outdoor activities and were walking along the trail at the time of their disappearance, were held on 19 February. Their families alerted the local sheriff's office when they did not return from their hike on Monday (13 February), with Abigail's mother posting a frantic message on social media when her daughter did not return, News.com reported.

She wrote: "If anyone has seen Abigail or Liberty German ... please contact myself or the sheriff's department immediately! The girls haven't been in contact for about three hours."

Police and volunteers searched for the girls, finding their bodies on Tuesday (14 February), with police saying there are currently 20 FBI officers on site investigating the case.

Citizens can provide information about this case by calling the Delphi Murder Tip Line at 844-459-5786. Information can be reported anonymously.