DELPHI, Ind. (WLS) --Indiana authorities are now offering a $50,000 reward for information leading to the killer of 13-year-old Abby Williams and 14-year-old Libby German.
Investigators released a short audio clip Wednesday that they believe is the voice of the person who killed the girls February 13.
In the audio clip, a male voice can be heard saying three words: "Down the hill."
The audio was posted on the Indiana State Police website.
Authorities say that audio clip was retrieved from Libby's cell phone.
Investigators revealed they have other video and evidence that was recovered from her phone as well. That evidence is not being released.
Investigators say it is unclear to them whether the voice heard in the audio clip is the same person seen in the still image of the suspect that officials have been circulating, or possibly another suspect.
"The audio quality is not superb, but there is enough there that someone can recognize the voice," said Indiana State Police Captain David Bursten.
Indiana State Police Sergeant Tony Slocum praised Libby for having the presence of mind to record what was happening during that horrifying situation.
"This young lady is a hero. There is no doubt. To have enough presence of mind to activate the video system on her cell phone to record what we believe is criminal behavior that is about to occur," he said.
Was this a chance encounter or did the suspect target the girls?
Bursten says all options are being investigated, adding this crime was one of two things.
"That was a chance encounter... or that person knew that they were going to be there," he said.
Authorities spent a big part of the Wednesday press conference delivering an impassioned appeal to the public to call in tips to help nab the person or persons responsible for the murders.
Investigators believe someone out there that recognizes the suspect can be the key to capturing him.
"Even with technology we need human intelligence, and we need you," said Indiana State Police Superintendent Doug Carter.
"Somebody knows. And if you're watching, we'll find you," Carter said, addressing the suspect.
Authorities and community leaders are now offering a $50,000 reward for any information leading to an arrest. The reward was increased by $9,000 Thursday due a donation from Rep. Todd Rokita.
They have set up both a phone line and email address for tips:
Police have received approximately 3,900 tips via phone and email. Approximately 1,900 of the tips were received after the release of the voice recovered from Liberty's phone.
The FBI is utilizing approximately 6,000 electronic billboards in 46 states requesting information about this case.