Investigators say Alex Cox’s phone points them to missing kids J.J. and Tylee: Part 9

The FBI’s cellular analysis survey team was able to place Cox’s phone at Chad Daybell’s property during key times related to the disappearance of Lori Vallow’s children.
6:53 | 02/27/21

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Transcript for Investigators say Alex Cox’s phone points them to missing kids J.J. and Tylee: Part 9
We've been covering this story for so long, and throughout every iteration, the question always remained the same -- where are the kids? I think for the public, they wondered, why were Lori and Chad allowed to walk free for so long? And now we finally understand why. Investigators needed time. When you look at the affidavit, you see how much work they put into this to try to piece it all together. Is that standard, to let people go about their business while the investigation's under way, even if they think murders took place? Yes, and mainly, they let that happen because they'll make mistakes typically along the way. They'll make phone calls. They'll go bury something. They'll get rid of a piece of evidence. Investigators were able to put together a timeline of sorts. So, give us a sense of what law enforcement now believes happened. So, what happened is that local law enforcement got the FBI involved, and rightly so. They bring a lot of support to a case, and there is a team or a unit in the FBI called the cellular analysis survey team, and what that is is that they are very good at precisely marking where your phone has popped up. It's actually so good that they can put it roughly about 20 feet from where you are. So, it's really cool stuff. And so that cell phone location allows for police to really put the pieces of the puzzle together. You can place Alex in Lori's apartment at wee hours in the morning, and then the next thing you know, a few hours later, Alex's phone pings behind Chad's house. It allows investigators to understand where that phone was traveling and then extrapolate from there. Now, you can't say this was Alex the entire time with this phone. You don't know that for certain. But when you start adding it up, you say, this is his phone going from place to place. Then the phone goes back to his apartment. And for investigators, it leads -- and others to say, who else could have done this? You don't know for sure. But you are sort of making that extrapolation. Tylee, the last time she's seen is on September 8th. She disappears. Alex goes over to the apartment early morning hours of September 9th. This evidence is fascinating because it's the only time that Alex's phone is at any period in time in Lori's apartment between the hours of midnight and 6:00 A.M. Then at about 9:30, the phone pings from Chad's property, towards the east end. Then there were the text messages from Chad daybell at around the same time, correct? Right. According to the affidavit, a short time -- perhaps after Alex had left the property, Chad then texts his wife Tammy daybell saying, hey, there's something interesting going on back by the pet cemetery. There was a raccoon and I had to end up shooting it, and then I buried it in the pet cemetery. He also mentions burning some stuff in the fire pit. I think he made an excuse that it was going to get wet or whatever and he wanted to get rid of it. Was there ever a raccoon discovered buried on Chad daybell's property? No. Let's talk about Lori. About two weeks later, September 22nd, what is she saying, Ryan, to her friends at this point about her children? Well, she's talking about J.J. She's frustrated. She allegedly mentions to her friends that she thinks J.J. Has become a zombie, and she uses as proof of that the fact that he's watching TV while he's sitting still and that his vocabulary's increasing. And that's proof that he's in a zombie-like state? Yeah, that this isn't the J.J. That she knows. And investigators say that this isn't the only time she's mentioned zombies and her children. Investigators say they have other witnesses who heard Lori Vallow refer to her children as zombies. Right, and this becomes key when you talk about zombies in this context. It's -- it relates to the beliefs that Lori and that Chad hold as part of this new church called the church of the first born. Zombies are these people who are always considered to have dark spirits. They were no longer seen as human beings. They were seen as zombies. And so what that does is it tees the ball up in her mind, expressing to other people that these kids are now bad and that who they were -- who tylee was, who J.J. Was -- are not anymore. She's saying her kids have been possessed? Her kids are possessed. Right. Maybe look at it this way, that they have created a different reality. On September 22nd, that very day that Lori was complaining to her friends, something happens. Alex comes over, yes? Alex comes over to the apartment. He takes J.J. He has him for a while, brings him back early that evening. There were house guests there. And so the next day, the house guests get up and there's no J.J. There are. So they say to Lori, hey, where's J.J.? And Lori allegedly tells the house guests that J.J. Was acting up, so Alex came back over and took him back in the middle of the night. And then j.j.'s never seen again. And in this particular story, investigators were able to use Alex's phone again to retrace where he actually went during that period of time. Alex is only seen, from his phone in the month of September, as being on Chad daybell's property four times. Two times, he's indoors, inside the property. But on the 9th and 23rd of September, he's outside in the backyard according to that phone signal. And those are the only times? Yep. I think one thing that makes this case difficult for investigators is it's hard to know exactly who did what. The signals will tell you where certain devices were, but what's been probably very difficult for investigators, and especially prosecutors in this case -- and when people look at this and say, well, why aren't there murder charges? Because you don't know exactly who did what. Who was involved in actually laying hands on these children? That search warrant executed at Chad daybell's house, what did they find? And your books are set for the month!

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

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