Teens' unsolved killings are 'open wound' for grandfather on anniversary of mysterious deaths

— -- One year after the mysterious killings of Indiana eighth-graders Abby Williams and Libby German, the grandfather of one of the girls says the past 12 months were a "blurry haze of existence" as he's "consumed" with "trying to catch this guy."

The unsolved deaths is an "open wound" that "can't heal until we get the justice for these girls that they deserve," Libby's grandfather, Mike Patty, told ABC News.

"Somebody took them way too early in life," he said. "They didn't ask for this, they didn't deserve this. Somebody chose to do something horrible."

A mysterious killing

Abby, 13, and Libby, 14, who became good friends as volleyball teammates, were enjoying a day off from school on Feb. 13, 2017, when they vanished on a hiking trail near their rural hometown of Delphi. Their bodies were found the next day about a mile away from where authorities say they were led off the trail.

No arrests have been made.

As the months have ticked by, clues in the mysterious double killing have been few and far between.

Police have not released how the girls died.

Authorities initially published a grainy image of who they say is the prime suspect in the investigation and broadcast a chilling recording found on Libby's phone with just three audible words: "Down the hill."

Police later released a composite sketch of a man believed to be connected to the killings.

'It's not a cold case'

Twelve months later, Indiana State Police Sgt. Kim Riley stressed to ABC News, "It's not a cold case. We are still very actively working it. We're still receiving tips as we speak."

"We are looking at probably close to 1,000 different subjects," Riley said, adding there could be one suspect or more than one.

About eight to 15 police officers are working the case every day and they receive roughly four to five tips each day, Riley said.

In the past year, authorities have received over 16,000 tips from across the United States and other countries, Riley said.

For Carroll County Sheriff Tobe Leazenby, whose office is leading the investigation, this is "definitely the biggest case" in his 32 years at the sheriff's office.

Leazenby told ABC News he is surprised the case hasn't been solved, admitting "there is some frustration." But he added, "we continue to move forward."

A killer in the community?

In this tight-knit community, dubbed by the local sheriff as "small-town USA," Main Street runs through the center, with the jail, city courthouse and a cozy library all located within walking distance of each other. A network of trails surrounds Delphi, and here, everyone knows everyone.

Riley said residents of Delphi "need to be very vigilant," because "whoever this person or persons are, they are still out there, as far as we are concerned."

Leazenby -- who said he's noticed residents contacting the sheriff's office more often to report suspicious vehicles or suspicious people -- urges locals to not give in to fear.

"If we bow to that, we've kind of given up," he said.

Greg Briles, the superintendent of schools at the Delphi Community School Corporation, also noted that community members seem "more aware of their surroundings," but he doesn't think the suspect is someone in their small town.

"If the person responsible actually lives in this community, they've done a heck of a job concealing themselves," Briles said.

Like the sheriff, Riley said it's "frustrating" that the girls' killer isn't in custody, but he vowed that police will continue to investigate until an arrest is made.

"We're not going to give up until we get this person or persons," Riley said. "Whoever the people are that were involved in this case -- we will get you. We're not going to stop until we do."

'Close that chapter in the book'

As the investigation inches forward, two grieving families are desperately waiting for answers.

Libby, who lived with her grandparents, had a gift for volleyball, softball and soccer, and was "a positive person that would have made a difference in this world," according to her grandmother, Becky Patty.

"We've lost a piece of our family," Mike Patty told ABC News this week, describing their home as "like a table without a leg."

"They're not here to make their mark in life, to grow up and to go to college ... get married, have families of their own if they chose to," Mike Patty said. "This sick person ... he not only stole their lives, he stole their lives from us."

But he's still confident police will make an arrest, which he says will allow him to "close that chapter in the book and start our healing process."

Abby's mother, Anna Williams, has described Abby as a happy child who was passionate about band and volleyball. Abby was "very respectful, very kindhearted," her mother told ABC News last year.

Williams, who did not immediately respond to a request for comment for this story, told ABC News in December she was "still 100 percent positive that something will come out of all of this, and that we'll find out who did this and we'll have our answers."

"We're still continuing to hope that the right person is going to do the right thing," Williams said, "and that tip is going to get called in, and we'll finally be able to move on to the next chapter."

Authorities urge anyone with information to come forward and call the tip line at (844) 459-5786.