Police line do not cross
File photo: US police tape Reuters

Police investigating the double murder of two teenage girls in Indiana are exploring the possibility that the killer could have struck before.

Liberty German, 14, and Abigail Williams, 13, were killed after going on a hike along a nature trail near their hometown of Delphi on 13 February, sharing a snapchat photo of their walk before they disappeared.

Their bodies were later found close to a disused railway bridge by the trail on 14 February.

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.

And detectives looking into the case have unearthed disturbing parallels between the killing of German and Williams and an unsolved double murder from 2012, in which 10-year-old Lyric Cook and her cousin Elizabeth Collins, 8, were also killed in Indiana, Fox News reported.

The killings happened just several hundred of miles apart. Both sets of girls were killed while on nature trails and disappeared on 13th day of the month, with Cook and Collins disappearing as they rode their bikes on an Evansdale trail on 13 July in 2012, although their bodies were not discovered until December that year.

Following the recent deaths of German and Williams, the father of Collins contacted police in Delphi to alert them to the similarities of the cases – which police confirmed they were already looking into.

Drew Collins told KWLL: "I wanted to make sure they were looking at it and they were already ten steps ahead of me, looking at it. They contacted Indiana State Police.

"My heart just breaks for the families. I don't know what they are going through. I know what I went through and our family went through. It is just very hard, it doesn't get easier."

Police have released a picture of a man spotted walking by the Delphi trail when Williams and German disappeared, announcing he is being treated as the main suspect in the case, also releasing audio recording taken from German's phone which features a man saying "down the hill".

Police said there could be more than one suspect and that the audio recorded on German's phone was not necessarily the voice of the man in the photograph.

"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."

The parents of German and Williams are set to speak to the press on Thursday (8 March).