DELPHI, IN – It was Valentine's Day, 2017, when the corpses of two young teen girls, Liberty "Libby"German and Abigail "Abby" Williams, were found under a train bridge.

One reason this particular homicide case came to national attention is the haunting cell phone and social-media clues the girls left behind. Just after 2 P.M. on February 13, Libby posted a photo to Snapchat of Abby walking on the bridge. In the distance, if you look closely, you can see a man approaching.

A closer photo of a man walking, perhaps the same man, was also found on Libby's phone and released, in hopes of a lead in the case. A composite of photos of this man and of other photos of the bridge area have been turned into a composite video and hosted on AbbyAndLibby.org, giving the impression of the suspect walking on the bridge.

But it wasn't just photographic evidence the girls left behind. Investigators also discovered an audio recording on Libby's phone, of a man with a deep voice, and they've released a section of it in which he is saying, "Down the hill."

The fact that Libby had the presence of mind to do what she could to implicate the man who at that point, must have been scaring and threatening the girls, is one reason this case has captured the public's attention. But despite Libby's heroic efforts to preserve evidence that could implicate her and Abigail's killer, the case remains unsolved.

In 2017, investigators released a sketch of a man they believed could have possibly been involved, but in 2019, the sketch was updated. Now authorities say the suspect could be between the ages of 18 - 40 and it's possible he's "hiding in plain sight."

Callahan Walsh told Inside Edition, "I went to the crime scene and I traversed that bridge, that railroad trestle myself. It's treacherous. It's very high off the ground. There's no handrails, there's planks missing,” Callahan said. “That is something that an individual would need to have experienced crossing to get over. And that person who followed Abby and Libby across the bridge made it across with no problem. It makes me believe that that person knew that area well, that (they) knew that terrain, because it's very difficult terrain, and had experience in that area.”

The families of Libby and Abby and the police are still working to find the girls' killer.

Read more: Inside Edition