Witness in Delphi double murder likely afraid, investigator says

Indiana State Police Superintendent Doug Carter told "GMA" investigators are "one tip away from success" in solving the murders of Abby Williams and Libby German.
5:02 | 05/15/19

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Transcript for Witness in Delphi double murder likely afraid, investigator says
The latest on the delphi murder mystery. In an exclusive interview the head of the Indiana state police is sending a new message to the killer of the two girls and Alex Perez brings us that from delphi, Indiana. Good morning, Alex. Reporter: Hey, good morning, George. Investigators have been searching for Abby's Libby's killer and they are one tip, one step away from cracking this case, they believe. I think of Abby and Libby all the time. I have their names etched in my desk. Reporter: This morning new details from Indiana's top cop on the hunt for whoever brutally murdered Abby Williams and Libby German. In a case that garnered national headlines for two years now and gripped this small community with fear. I want them to know as long as I'm the superintendent of the state police in Indiana we're not leaving. We're going to stay there and when we run out of tips we'll start over again. Reporter: In an ABC news exclusive Indiana state police superintendent Doug Carter revealing investigators received more than 3,000 tips following that emotional press conference in April where he addressed the girls' killer directly. To the killer who may be in this room, we believe you are hiding in plain sight. Reporter: Carter chose that direct approach because he believes the updated sketch he received that day will strike fear in the heart 69 killer. I don't believe the individual knew we were going to do that so it was really, really important and I think that he was probably there and/or watching simply because he thought we were on the wrong path. Reporter: Along with this extended clip of the killer's voice including the word "Guys" captured on Libby's cell phone. Guys, down the hill. Reporter: Carter is hopeful someone in delphi now has enough to recognize and identify the killer. Guys, down the hill. Guy, down the hill. That extra word, what are you hoping that extra word is going to show or demonstrate or why release that. Somebody has heard him say that before. He just didn't pick that word. He had used it before. He had used it before and it might be just that one word that makes somebody think, gosh, I do know that voice. Reporter: Whoever that is, he hopes they'll find the courage to come forward. Somebody knows who that person is. If somebody knows something, they need to tell us. I think they're afraid to is why I think we have not heard yet. Reporter: And for that suspect Carter warns the cops know much more than they can share and that one tiny clue could bring an end to all of this. We are one tip away from success. We are one tip away, one bit of information away from finding out who this individual is. Reporter: And superintendent Carter tells me they have received more than 42,000 tips since this all started. He says he thinks about Abby, Libby and solving this case every single day. George. Okay, Alex, thanks very much. We're joined by John Douglas. Thanks for coming in. That superintendent seems determined. Thinks he's close. What do you make of his strategy. The strategy is good. He should have done the strategy a long time ago. This case really could have been a solvable case really quick. I learned over the years at quantico and working these cases, you should be releasing information relative to pre-offense behavior, post-offense behavior regarding the crime. The tape itself, it's not an order he's giving them. It almost sounds like a schoolteacher saying, guys, down the hill or a coach like swisher -- Very familiar. Familiarity with the area. When they perpetrate a crime there is the comfort zone where they don't just perpetrate a crime particularly like this one in an area they've never been so this person has been there and feels extremely comfortable. It was -- the victims were low risk victims until they decided to go out in that isolated morning and increased their risk level. On the part of the offender, his risk level was pretty high because it was daylight. This was a park. It's public and there was a really strong chance someone could have come upon him. What kind of profile would you be building based on the information you have and what additional information would you need to make it more accurate? Well, what you need is the autopsy protocol, the autopsy photographs, the method and manner of death. You do the victimology, which is important. In this case as I say they're very, very low risk types of victims. So you have this information and for an age group I usually start off at about mid-20s to late 20s. I've seen that the bureau came out with a rank that's way too long from 18 years of age to 40 years of age as a potential suspect. I start in the mid-20s and either tack on years or eliminate years based upon either the sophistication of the crime or lack of sophistication, you know, of the crime is there let's hope we stay close on this. John Douglas, thanks very much

This transcript has been automatically generated and may not be 100% accurate.

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