here's abc's amy robach. >> reporter: this week, justice in a new jersey courtroom. >> guilty. guilty. guilty. >> reporter: leah, a former friend of sarah stern, found guilty on all counts of murdering the 19-year-old who went missing from her home in december 2016. a jury determining that he strangled her and enlisted her junior prom date, preston, to help throw her body off a nearby bridge. >> to think that somebody who pretended to be her friend actually did that to her and without any remorse. it's just, it's painful, it's sad, and it's just, it's beyond anything that i've ever experienced before in my life. >> reporter: prosecutors say the motive, getting their hands on sarah's lock box, which he believed was stuffed with up to $100,000 in cash. >> i picked her up under the shoulders and hoisted her up. >> reporter: and now newly-released video shows that accomplice, preston taylor, walking investigators through exactly what happened after the murder. >> i carried her over here. dragged her over here and kind of sat her under the bushes right here. >> 911, where's the emer

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i'm david muir. >> and i'm amy robach. this is "20/20." it was one of the biggest stories of the year. the girl who went from classroom to captive. >> and we haven't heard from the young woman who went missing. but that all changes here tonight. here's eva pilgrim, on the story from the start. >> reporter: tonight, we are heading to rural culleoka, tennessee, where this strange and heart-wrenching story begins. >> most people know everyone else who lives there. if there are six degrees of separation in the world, there's only about one degree of separation here. >> reporter: in this farming community of about 5,000, you'll find a post office, gas station, and not much else. >> it's very small. very small there. nothing around in. >> reporter: until, of course, it does. >> at the center of on nationwide amber alert. >> reporter: it's been a year and half now since 50-year-old high schoolteacher tad cummins ran off with his 15-year-old student. today, some squarely blame him. >> should be drug out in the streets and beat. >>

here's abc's amy robach. >> she goes to walk out the front door. i choke her out. drag h. my b problem was the dog. and her dog laid there and watched as i killed her. didn't do anything. [ bleep ] dog. and it taook me a half hour to kill her. i thought i weighs goias going e to choke her in a few minutes. i got a shirt and shoved it down her throat so she wouldn't throw up or anything. >> reporter: a friend's shocking confession. >> you are the only person on this planet that knows. >> reporter: all caught on tape. >> heartbreaking, somebody that's your best friend, you don't think they would do something like that. >> the worst part of it was i thought i was walking out with $50,000, $100,000. she had a safe and only had $10,000. remember o >> reporter: one man's decision to secretly record his friend. >> had he not come forward we probably wouldn't have gotten an answer. >> it's now exactly two months to the day since police found sarah stern's abandoned car up on that bridge. >> her body not yet found but tonight cops say she was killed by one of her friends. >> report

later, laughing it off in an interview with abc's amy robach. >> i'm also going through menopause. so i didn't think my costume was hot when i first tried it on. >> reporter: last year she was still laughing when she explained away her absence on the air. >>ive abeen trending regarding, what the hell is going on with wendy. >> reporter: this time citing doctor's orders with graves disease and >> it was a perfect storm since late july. >> reporter: late last year again, citing a shoulder injury. >> i have a hair fracture on my right shoulder, and i did it on sunday, and by monday morning my shoulder was on fire. >> reporter: and finally, this year, a mysterious two-month absence from her show. a chorus of fill-in hosts but no explanation, but then today. >> i am driven by my 24-hour sober coach back to a home that i live in here in the south, i mean in the tri state with a bunch of smelly boys who've become my family. doors locked by 10:00 p.m. lights out by 10:00 p.m. so i go to my room. and i stare at the ceiling, and i fall asleep to wake up to come back here to see you. >> addic

here's amy robach. >> reporter: it was a shocking murder confession. >> it took me a half an hour toill her. >> reporter: liam mcatasney describing how he strangled his friend, 19-year-old sarah stern. >> she goes to walk out the front door. i choke her out, drag her. >> reporter: the friend who secretly helped police record that confession -- anthony curry, now speaking out for the first time. >> it's heartbreaking in a sense, you know. somebody who's your good friend. you don't think they would do something like that. >> reporter: this newly released video revealing more chilling details. >> i grabbed her legs. we got her over the fence. >> reporter: stern's junior prom date, preston taylor, here showing police how he helped mcatasney dump stern's body off a bridge. >> liam had her by the shoulders and hoisted her up onto the railing, and then i pushed her feet over so she was going over the rest of the way. >> reporter: their ultimate goal? steal stern's inheritance money locked in a safe. mcatasney's own words sealing his fate this week in a new jersey courtroom. >> guilty. >> gu

here's abc's, amy robach. >> reporter: in the new hit horror thriller, "us," lupita nyongo plays both good, and evil. it's one of her character's voices that is getting attention. >> be careful. >> reporter: the 36-year-old actress said to tap into a sense of trauma and fear she turned to a rare disability called spasmodic dysphonia and affects some 50,000 people in the u.s., impairing their speech. >> when they speak, certain words trigger their voicebox to slam shut. >> reporter: she spoke about the condition to "variety" at the premier. >> it creates spasming in the vocal cords, and i worked with an ear, nose and throat doctor a vocal therapist and dialect coach to make sure i could do it and do it safely. >> reporter: but some who live with the condition aren't impressed. one tweeting, i so appreciate my condition being used for your evil character, not. >> i think the biggest challenge is that it's being associated with the terms creepy and haunting. and the reality is once the movie is over they are still living with this voice disorder. >> reporter: she says she was partially i

last march, williams talked to amy robach about her diagnosis and her hard-charging schedule. >> in thethe show i've never taken a sick day. >> people do tend to return to work prematurely when they're celebrities. i hope in this case she's able to maintain her sobriety as a priority. if she can do that, great, then this should be no problem returning to work. >> williams is getting an outpouring of support from her fans, some even sharing their own experiences. she offered few details about her current abuse and there's no doubt that there's probably more to come about her struggles. >> and she will share it, all right. thank you so much, deb. >>> and we have larry hackett here, the media and entertainment expert and our dear friend. right after the show we're back in our dressing rooms and we have "the wendy williams show" on and we're huge supporters of hers and she's taking control of her story. >> she has and created now a story that people will be interested in. as deborah said she was forced to do this because there was a tabloid report coming out about where she was staying and