This is the prime suspect in the murders of Indiana teenagers Abigail Williams and Liberty German, police said Wednesday - recorded by Liberty before her death.

Authorities had released the picture last week saying they were looking to speak to the man but stopped short of calling him a suspect.

Police said on Wednesday that Liberty, 14, had taken the picture of the man on her cell phone.

Police in Delphi say this man - who was pictured near to the Monon Bridge where Abigail Williams and Liberty German started their walk - is the main suspect in their murder, and that Liberty recorded his voice too

The bodies of Abigail (left) and Liberty (right) were found dead about three-quarters of a mile from an abandoned railroad bridge on the Monon High Bridge Trail on Tuesday last week

Revelation: Indiana State Police Superintendent Doug Carter said the voice of the killer was a key clue which he hopes someone would recognize

Libby posted these Snapchat photos of Abigail at Monon High Bridge Trail close to when they started hiking around 2.07pm Monday, 13 February. They are the last photos of the girl. Both were founded murdered three-quarters of a mile away the next day

An audio file from the 14-year-old's cell phone was also released with police saying that she had 'the presence of mind' to turn her video camera on.

Indiana State Police Sgt. Tony Slocum said: 'This young lady is a hero. There's no doubt.'

The tape consisted of three words: 'Down the hill'. Police asked for the public's help in identifying the voice.

Police said that it was possible the voice belongs to the man in the photo but also said more than one person could be involved.

On Wednesday morning, police appealed to the public to come forward with any information on the man who was seen on the abandoned train tracks on the Delphi Historic Trails at the same time on February 13 when Williams, 13, and German went missing on a hike.

The two friends were active on social media on their phones before they disappeared, with German posting final Snapchat images of Williams walking on the bridge, which is 60 feet above the creek, at 2:07pm.

In the grainy image, the man, can be seen walking with his head down and hands in his pockets, along the same bridge. He is dressed in jeans, blue jacket and a hat.

Sgt. Slocum said: 'We are actively looking for this person. This person is our suspect.'

Authorities believe either the victims came across the killer by chance or the suspect knew their plans and was lying in wait.

Appeal: Law enforcement, including Captain David Bursten of the Indiana State Police (at the podium) released the new recording on Wednesday in Delphi

This is the bridge where Abigail Williams, 13, and Liberty German, 14 were last seen alive on Monday, thanks to the Snapchat posts

How last week's investigation unfolded: The girls went ona hike on Monday 13 February.

Indiana State Police Superintendent Doug Carter added: 'Someone knows who this individual is… And if you're watching, we'll find you.'

He also reminded the public that tips could be submitted anonymously.

Abigail Williams, 13, and Liberty German, 14, went missing last Monday after going for a hike at 1pm near the Monon High Bridge along the Delphi Historic Trails.

German had last posted a Snapchat image on the bridge shortly afterwards, the last time they were known to be alive.

They were reported missing last Monday afternoon when they failed to meet up with a family member at a prearranged location.

Their bodies were found on February 14 at 12:15pm around 60 feet from the creek, about three-quarters of a mile upstream from where they were last seen.

A $41,000 reward has been put up in the case, using funds from the FBI, a local business and a local family.

Indiana is a death penalty state.

The 24-hour tip line is 844 -459-5786.