DELPHI, Ind. -- Police say one of the two teenage girls killed last week after going for a hike in northern Indiana recorded the voice of a suspect in their deaths.

Indiana State Police played a clip of a man’s voice saying “down the hill” during a Wednesday news conference where officials appealed for more tips from the public and announced that $41,000 in reward money had been raised.

Audio released in connection with Delphi teens homicide case Authorities released this short audio clip in connection with the Delphi, Indiana teens homicide case. In the audio file, a man can be heard saying "Down the hill." Police said the photo of the suspect came from 14-year-old Liberty German's phone. The bodies of Liberty, and her friend 13-year-old Abigail Williams, were found on Feb. 14 about a quarter-mile from an abandoned railroad bridge. Officials stopped short of saying the voice in the recording was of the suspect seen in the photo. However, they hope someone can either recognize the man's voice or identify the suspect seen in the picture. Authorities are confident they will find the killer and hope anyone with information will come forward. Share this post to spread the word. http://via.cbs4indy.com/Fp108 Posted by CBS4 Indy on Wednesday, February 22, 2017

Capt. David Bursten says the audio came from 14-year-old Liberty German’s cellphone. Bursten says Liberty also captured an image of a man released by police last week of a man walking near the hiking trail outside Delphi about the time she and 13-year-old Abigail Williams disappeared Feb. 13, Bursten said.

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Bursten said investigators aren’t certain the voice was that of the photographed man who investigators are calling the main suspect in the deaths. That man hasn’t been identified and no arrests have been made. Officials haven’t ruled out the possibility that more than one person was involved.

The girls’ bodies were found Feb. 14 in a rugged wooded area off the trail some 60 miles northwest of Indianapolis. The remains were about about a quarter-mile from an abandoned railroad bridge that’s part of a trail system where the teens planned to go hiking during a day off school in Delphi, a rural community of about 3,000 people.

The audio quality is “not superb, but there’s enough there that somebody could recognize this person’s voice,” Bursten said.

Officials declined to release more information about how the girls were killed and what else Liberty might have recorded on her phone. Bursten investigators have more video relevant to the investigation they are not releasing.

They hailed German as a hero.

“This young lady’s a hero, there’s no doubt, to have the presence of mind to activate the video on her cell phone to record what we believe is criminal behavior that is about to occur,” said Indiana State Police Sgt. Tony Slocum.

Greg Massa, assistant special agent in charge of the FBI’s Indianapolis Division, said as many as 20 FBI agents have been assisting state and local investigators.

Indiana State Police Superintendent Doug Carter became emotional when he spoke at a press conference Wednesday.

“Why Libby? Why Abby? Why Delphi? Why Carroll County?” he said. “Why the region, why the state, why even in the nation? I say that because this is a classic example and a clear example that evil lives among us.”

But Carter vowed, “We’re not stopping. Someone knows who this individual is.”

He also spoke directly to the girls’ killer.

“If you’re watching, we’ll find you,” Carter said.

The police tip line for the case is: 844-459-5786.