A rural community where two young girls were found murdered near an abandoned railway on Valentine's Day remains gripped by fear and paranoia as the person who killed them continues to evade capture.

Abigail "Abby" Williams, 13, and Liberty "Libby" German, 14, were reported missing on February 13 after they were dropped off to go hiking near the Monon High Bridge in Delphi, Indiana, and didn't return.

Their bodies were found the following day in a wooded area near the Delphi Historic Trail. The case has been dubbed "The Snapchat Murders" because of two haunting photographs posted to the social media platform by Libby before the friends were murdered.

The 14-year-old later captured footage of the suspected killer, who is seen walking briskly along the rickety rail bridge with his head bent and partially covered by a hoodie.

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Police believe Libby starting filming after becoming concerned the man was following them and there has been speculation on social media that he is the dark shape seen in the distance of her final Snapchat of Abby.

Officers also released looped audio of what is believed to be the killer saying "down the hill", in a gruff, authoritative tone in the hope that someone will recognise the distinctive voice.

Photo / Supplied
Photo / Supplied

Indiana State Police have not ruled out the possibility of more than one suspect and said it was unclear whether the voice in the audio clip belongs to the man in the photograph or another.

"Someone knows who this individual is," an emotional Indiana State Police Superintendent Douglas Carter told reporters at a recent press conference. "And if you're watching, we'll find you."

Authorities have said they recovered more footage from Libby's phone but declined to release it.

The FBI has been assisting local authorities in the investigation, with agents reportedly briefing Director James Comey on the case on at least two ocasions, according to ABC NEWS.

The bureau's Behavioural Analysis Unit - which has been brought into popular culture by American TV shows such as Criminal Minds, The Wire and serial killer classic The Silence of the Lambs - is currently building a profile of the killer.

Overnight, the reward for information leading to an arrest doubled to more than US$200,000 thanks to a US$100,000 donation from sporting identities Pat McAfee and Jim Irsay.

None of this activity is easing the fears of residents, especially families, who have had to explain to their children that a real life Boogeyman is on the loose.

Speculation is running rife among locals and on social media about the possibility the girls were catfished by somebody posing as a teenager that they met online or whether they were the random victims of a violent stranger.

They fear that as long as the perpetrator remains at large, he could strike again without warning.

Asked by the Lafayette Journal & Courier if she felt safe, Delphi local Jennifer Robinson said:

"Not particularly, to be honest with you, and I've got three kids," Robinson told the newspaper. "To me it's kind of a big deal."

Fellow resident Dennis Walker added: "There's a lot of blowing up going on at Facebook. A lot of angry people. A lot of other people saying we got to pray for the family, do what we can.

Liberty German posted these Snapchats of Abigail Williams at Monon High Bridge Trail close to where they started hiking around 2.07pm yesterday. Photo / Liberty German Snapchat
Liberty German posted these Snapchats of Abigail Williams at Monon High Bridge Trail close to where they started hiking around 2.07pm yesterday. Photo / Liberty German Snapchat

"A lot of people go out there for a little hiking, a nice day in the woods, get a little exercise. This shouldn't happen to you when you go out there."

Julia Leahy told the Journal-Courier, it's going to "take this community a long time" to get over this.

Melissa Schwartz told WRTV, "I think it's very scary, I was raised in Delphi, grew up in Delphi and we've never had anything like this happen in Delphi, so it's very concerning."

While police have not called it a homicide investigation, some in the community have jumped to conclusions.

"I would always be concerned about that until they find whatever murderous beast did this. I would watch my kids very carefully," Kevin Kologinski told WRTV. "Mine are all grown up, my grandchildren are too young to be off on their own, but for any other kids that are out I'd keep a very close eye."