'Epitome of evil': Delphi double murder still a mystery 3 years later Abby Williams, 13, and Libby German, 14, were killed on Feb. 13, 2017.

Delphi had been known as a quiet, tight-knit, rural area in central Indiana. Described by the sheriff as "small-town USA," with one main street running from the jail to the courthouse to the library, everyone seemed to know everyone.

But on Feb. 13, 2017, Delphi's obscurity was erased with the murders of eighth-graders Abby Williams and Libby German -- a crime Indiana State Police Superintendent Doug Carter calls "the epitome of evil."

"I'm in my 35th year and I've never experienced anything quite like it," Carter told ABC News.

Three years later, their slayings remain unsolved.

The mysterious crime not only rocked rural Delphi, it also captured the interest of the country -- and Carter has vowed to forge ahead until the killer is found.

Abby's mother, Anna Williams, is frustrated three years have gone by, but she says no one has seemed to forget about their case.

"I still have faith," Williams said. "Law enforcement has it under control. I believe in them 100%."

Hiking trail horror

On Feb. 13, 2017, Abby, 13, and Libby, 14, who had become fast friends as volleyball teammates, were enjoying a day off from school and went to walk along a local hiking trail.

But they never came home.

The girls' bodies were found the next day near the trail, several hundred yards from the Monon High Bridge, where they had posted this Snapchat photo.

How Abby and Libby died has still not been released to the public, which Carter said is "because only the killer knows."

Arika Gibson was 14 when her best friends were killed. Now, she's a junior in high school. Looking back on these three years without Abby and Libby, Arika told ABC News her grief is "an everyday thing. I'm still thinking about them in everything I do."

Abby was raised by her single mother, Anna Williams, and the two had an especially close relationship.

Abby and Libby were only in eighth grade, but had already talked about pursuing careers in forensic science, Anna Williams said.

Libby, a gifted athlete, lived with her grandparents, who had been her guardians since she was 3 years old.

They described their granddaughter as wise beyond her years.

"Libby already had her career path pretty much laid out. She wanted to work with the FBI, she wanted to solve crimes, she wanted to help people," her grandmother, Becky Patty, told ABC News. "If anybody wants to honor her, please, help solve her crime."

'Let us know what you know'

Soon after the murders, authorities released a grainy image of the suspect, who they say was on the trail the day the girls went missing. State police last year released video footage from Libby's phone. The brief video clip shows a grainy image of the suspect walking on the bridge near where the girls were last seen.

Police also publicized the suspect's voice -- a recording of him saying "down the hill" -- which was recovered from Libby's phone.

Last year, police released a new suspect sketch that Carter says was based on a witness' recollection of what he or she saw.

Carter said that he believes that since the killer's image and voice were publicized, and they have not been able to identify a suspect, someone knows who he is but is keeping quiet because of "extreme fear."

"There is obviously someone withholding information," added Carroll County Sheriff Tobe Leazenby. "It could very well be somewhere along the lines someone was even threatened not to reveal the identity of the killer."

"Please, please, come forward and let us know what you know," Carter pleaded.

48,000 tips and counting

For Libby's grandfather, the three-year mark is just "another year with no answers" and "no resolution."

"He could be anywhere," Mike Patty said of the killer.

About 48,000 tips have poured in over these three years from around the country, said Carter.

"I'm still constantly thinking about people that I know around town that I see," added Arika, the girls' friend. "I personally have not turned in tips, but I've told people to turn in tips when they're suspicious."

Leazenby wouldn't discuss the evidence or details of the investigation, but said "our investigators still have things they're following, whether it's new information or going back and looking at certain pieces from the past."

The sheriff said he thinks the killer has a connection to Delphi, "whether it's an individual that previously lived in our community and knows the area where the girls were located very well, or possibly is still in our community."

Carter also believes the killer has a connection to Delphi, and ABC News contributor and former FBI agent Brad Garrett agrees, noting that the hiking trail is probably too remote to attract outsiders.

"Unless someone absolutely somehow studied that [trail] in great depth, they would not know specifically where certain things were," added the sheriff.

And since the suspect may be a local, he could have already been interviewed when investigators canvassed those in town, Garrett said.

"One of the keys in cases like this is going back and re-interviewing people that you've interviewed before," Garrett said, because "sometimes people's stories change, you find inconsistencies. Sometimes people are mad at each other, they have a fight, and so they're more inclined to tell you a piece of information that they didn't two years before."

But this case also has some limitations, according to Garrett, who said the hiking trail is too far off the beaten path for police to track people's movements there by traffic and surveillance cameras.

As for Carter, he insists the case is "still very active" and vows it'll "never be cold as long as I'm breathing."

Anyone with information is asked to contact abbyandlibbytip@cacoshrf.com.