"20/20" continues with unfriended. Here again, Ryan smith. Reporter: Police have begun to suspect that Shelia and Rachel, best friends of missing 16-year-old Skylar neese are lying about what happened... See More
"20/20" continues with unfriended. Here again, Ryan smith. Reporter: Police have begun to suspect that Shelia and Rachel, best friends of missing 16-year-old Skylar neese are lying about what happened the night she vanished. The first red flag -- tension in the relationship. Officer Jessica Colebank pores through Skylar's old Facebook and Twitter posts. What she finds sets off alarms. You could tell she was mad at someone. Reporter: In the weeks before she vanished, Skylar had posted angry messages. "There's just something about you I can't stand." "People can be so mean for absolutely no reason." "Hope you don't expect me to give a Anymore. #Bye." And the final tweet she wrote, sent 10:48 P.M. The night she disappeared, "You doing Like that is why I will never completely trust you." She didn't trust someone, that was the issue. Reporter: And perhaps the most suspicious posting by Skylar, this strange game among best friends. Shelia asks Rachel and Skylar how they prefer to die. Would you guys rather suffocate or get shot? Get shot. Shot. Drowning or suffocating? Suffocating. It's almost the same thing. I know but it's not. Reporter: And when police review Rachel and Shelia's cell phone messages after Skyler went missing, they note a curious absence. There was no mention of Skylar whatsoever. It was very odd. It's as if she just erased it from her, from her mind forever. Reporter: Skylar has been missing for months, despite her parents concerns about Shelia's bad influence, they're convinced that Shelia is simply grieving over her lost best friend. Even as the police are saying to you they've got some suspicions about Shelia, you were thinking, hey, back off of this girl. Oh, I even told them. I said, "Will you please leave her alone? She's going through enough." I said, "Those girls have just lost their best friend. Will you chill out on her for a while?" Reporter: What the neeses don't know is that police are finding actual evidence of the girls' deception. Shelia and Rachel told police their joyride with Skylar was contained to the star city area. Desperate for clues, police scoured surveillance video from local businesses. It paid off. This video captures Shelia's car at 12:39 A.M. Heading north, away from star city. That was the first variation in their stories, is where they drove around. Reporter: And there's not just video. Cell tower pings from that night place them in the backwoods town of blacksville. That just blew their whole, oh, we stayed in star city. You're 45 minutes away from where you're saying you're at. You can't do that. You're not houdini. Reporter: So, you say, aha, we have this information. What happens when you speak to Rachel and Sheila? They change their story to fit, oh sure, we were in blacksville, I forgot that. Oops. Reporter: The carefully rehearsed stories are now mismatched. Shelia, the high school drama queen, claims Skylar didn't join them for the ride to blacksville. But Rachel, the high school drama star, says she did. They're starting to differ, at least from Rachel's perspective, it sounds like the actress is starting to forget her lines. Oh, yeah. She had to come up with her own line and that's where she really started tripping up. And once you start seeing inconsistencies in a story, it's all over. The person's lying. And then you must ask the question, why would they lie? Reporter: Shelia is adamant that her version of the night's events is true. She volunteered to take a polygraph. What did that say to you? Arrogance. I think she thought she was smart enough to outsmart a machine. Reporter: So, she took the polygraph, and what happened? Failed miserably. Reporter: Rachel also agrees to take a polygraph. But en route on the day of test, she gets scared, jumps out of her father's car and flees. She jumped out of a moving vehicle to avoid it. If you haven't done anything wrong, why won't you take this test? Reporter: Now, armed with surveillance video and cell phone records that prove the girls have been lying, police tell Dave and Mary neese that Skylar's best friends have a secret. At that point we knew they had to know something. So we pushed. Reporter: Your mission is to try to get their secrets out of them. Exactly. Reporter: The neeses start posting about karma on Facebook, writing that the girls need to tell what they know. Classmates and even strangers are putting pressure on Shelia and Rachel. Anonymous tweets like this one -- "Pretty little liars keep on lying!" All eyes were on Sheila and Rachel. You know, you can't be part of the trio and not have any clue what's going on. Reporter: Kelly Kerns, close friends with Rachel and her mother, hears the rumors that Rachel is stonewalling police. I'd get a text from people, why isn't your friend's daughter cooperating, so I'd send Rachel a text. What the heck are you doing? Reporter: Even those closest to Rachel are wondering, what is she hiding? I said, was Skylar alive when you left her? Yes. We didn't do anything to her. I think that in this situation, you cannot lessen the importance of peer pressure. Reporter: Rachel seems particularly affected. As the lead in that year's school play, she is accustomed to adulation. It's like she was getting booed in a performance. It's that type of level of ostracizing her. She couldn't handle that. Reporter: One night, almost six months after Skylar went missing Rachel cracks, apparently having a nervous breakdown. Her mother calls 911. I have an issue with a 16-year-old daughter of mine. No, this is over! Hurry up. Oh, god. Hurry up, please. Reporter: Seemingly suicidal, Rachel is admitted into a local psychiatric hospital. What happens there has never been revealed, but immediately upon release Rachel is driven directly to meet with police. She was just nervous, she was shaking. It was like, now the truth's finally gonna come out.
This transcript has been automatically generated and may not be 100% accurate.