DELPHI, IN — On February 13, 2017, best friends Abby Williams, 13, and Liberty "Libby" German, 14, went out for a hike near their home in Delphi, Indiana — and never came back.

That Monday morning, the girls woke up at Libby’s grandparents house after a sleepover. They were reportedly excited about having a day off from school, and decided to go for a hike on a popular path nearby.

Libby’s older sister said she dropped the girls off at the hiking trail at around 1:30 P.M. At 2:07, Libby posted a photo on Snapchat of Abby crossing Monon High Bridge, an abandoned railroad bridge over Deer Creek.

Libby’s dad Derek said he went to pick up the girls at 3:30 – but they didn’t show up, and he was unable to reach Libby on her cell phone.

At around 5:30, the girls’ families said they called the sheriff’s office. Law enforcement joined friends, family, and volunteers, scouring the woods looking for the girls.

Police say they made the decision to suspend the search at 12 A.M., stating that there was no reason to suspect foul play. But friends and family continued to search throughout the night.

Libby's grandfather Mike Patty said he knew immediately that something was very wrong. "They would have walked into town and called. Libby was resilient enough ... she would not have just waited out there until somebody would have shown up," Mike said. "That was just not her."

Then, on February 14, searchers reportedly found a single shoe. Within minutes, they made another grim discovery — they reportedly found the girls' remains, less than a mile from where they had started their hike just 24 hours earlier. The tragedy made headlines around the country, and media flooded the small town.

Police began their investigation, and say they soon found chilling clues related to the girls’ last moments alive, that Libby managed to capture on her cell phone. On February 15, investigators released one of them: a grainy photograph of a man crossing the bridge, taken by Libby.

Authorities released another clip a few days later: a muffled audio recording of a voice saying “Down the hill.” Investigators wondered if the voice could have belonged to the man in the photo — but the identity of the strange man is still a mystery.

Police called Libby a “hero” for her quick thinking in capturing the photo and audio evidence. Her family agrees. “Those girls loved each other, they were good friends. They never left each others’ side. Those girls were heroes,” Patty said.

A sketch of a person of interest based on witness descriptions was released to the public, and over the years, police have followed several leads. In September 2017, police arrested a sex offender in Colorado who threatened hikers with a hatchet. Since the man resembled the sketch, investigators considered him a person of interest, but after interviewing him say they were able to rule him out.

In January 2019, another accused sex offender was arrested. Police said that his social media posts were full of reports of sex crimes. Once again, though, the lead appeared to be a dead end.

A few months later, police released a new sketch of the suspected murderer – as well as additional audio from Libby’s phone.

Indiana State Police Superintendent Douglas Carter addressed the killer directly at a press conference, where he also stated that the killer is believed to be between 18 and 40 years of age and either lives, works, or visits Delphi on a regular basis. "We believe you are hiding in plain sight" and even "may be in this room," Carter said. "We likely have interviewed you or someone close to you."

Police and families continue to search for answers, but the case remains unsolved.

Mike Patty told Inside Edition that every time a new potential person of interest is mentioned, the families go on an emotional roller-coaster. He says he believes that the right person being arrested and prosecuted could at least allow them to move forward.

"There'll never be closure," he told the news channel. "We've lost our girls. They're not coming back. But at least it would allow us to move on from this chapter to the next so we could all start to heal, because right now, it's just kind of an open wound."

To learn more about this case, watch Still A Mystery on ID GO now!

Read more: Inside Edition, CrimeFeed, CrimeFeed (2)