Courtesy Sarah Turney Sarah Turney holds a picture of her sister, Alissa, in this photo from early December
Sarah Turney had no reason not to trust her father.
She was 12 in 2001 when her 17-year-old half-sister, Alissa, suddenly vanished on the last day of her junior year at Paradise Valley High School in Phoenix, Arizona.
Alissa Turney, an average student with a steady boyfriend and a part-time job at the local Jack in the Box, had fantasized about running away, so when her father, Michael Turney, assured Sarah that the spirited teen with an affinity for animals, sketching, and hard rock left on her own, she believed him — especially after finding a note in Alissa's room saying she was headed to California.
"I wasn't worried," Sarah, now 31, tells PEOPLE in the latest issue, on newsstands Friday. "I was under the impression she was going to be back. I don't think her being gone forever was anything that ever crossed my mind."
For years, even as suspicion swirled around his possible involvement, Sarah defended Michael, a former sheriff's deputy and electrician.
Certain the investigation had stalled, she launched a website seeking tips on Alissa's whereabouts, appeared on local and national television news shows to discuss the case, and combed through thousands of pages of police records.
The more Sarah searched for answers, the more convinced she'd become the man who gave her life took her sister's life. "For me, it was kind of like a switch," explains Turney, a former events and marketing specialist who was 4 when Alissa's mother, Barbara Strahm, died from lung cancer.
Justice for Alissa Alissa Turney
Sarah would be the last of her five siblings to believe Michael's guilt, but was the only one intent on holding him accountable. Detectives at that time encouraged her to generate more publicity about the case, hoping it might provoke fresh leads. And Sarah did just that, starting countless social media pages dedicated to Alissa's disappearance and launching an online petition calling for Michael's arrest that has been signed by nearly 300,000 people.
Sarah also produced a successful podcast about the case and created a series of videos about Alissa's presumed murder for TikTok. Those clips have been seen by tens of millions of people. "I never underestimated the power of social media, I just never thought I'd win the TikTok lottery," gushes Sarah.
Sarah's podcast, "Voices for Justice," along with the viral TikTok videos, helped renew interest in the cold case, and on August 19, a grand jury indicted Michael Turney, 72, on a single count of second-degree murder.
As part of her podcast, Sarah included chilling audio from a 1997 home video, of Alissa calling Michael a "pervert." There's also audio of Michael from October 2017 that Sarah covertly recorded when the two met at a Phoenix-area Starbucks to talk about Alissa: The then 28-year-old is heard confronting her dad, asking him to finally come clean about what happened.
Maricopa County Sheriff's Office Michael Turney
"Be there at the deathbed, Sarah," Michael callously advised. "I will give you all the honest answers you want to hear." It was the last time Sarah spoke with her biological dad, and those audio clips helped police make a strong case for Michael's arrest.
For more on Alissa Turney's disappearance and her stepfather's arrest, subscribe now to PEOPLE, or pick up the latest issue, on newsstands Friday.
Police records show Alissa Turney poked her head into her boyfriend's workshop class around 11 a.m. on May 17, 2001, to tell him she was leaving school early. Michael, she said, was driving over to pick her up. Michael called the Phoenix Police hours later, and told them Alissa had run away, leaving behind her clothing, cell phone, makeup, jewelry and car.
Police opened a missing persons case but didn't drive to Michael's house to interview him or look at Alissa's room. Instead, by phone, Michael explained he'd taken Alissa out for lunch that afternoon, and the two had an argument prior to him dropping her off at home around 1 p.m.
The stepfather told cops Alissa likely went to go live with an aunt in California, but that aunt told relatives her niece never showed. Then, on May 24, Michael contacted police again, telling them Alissa had called him from a Riverside, California, pay phone, before immediately hanging up. "He always made me uncomfortable, from the moment I met him," confides Charity Behrend, 38, Alissa's longtime friend.
The focus of the investigation shifted in 2006, when serial killer Thomas Hymer falsely confessed to Alissa's murder. For the first time, detectives conducted interviews with Alissa's closest friends — her boyfriend, Jon Laakman, among them. Michael, they said, had been verbally abusive, calling Alissa "stupid" and a "moron." They also dropped the bombshell allegation that he had sexually abused her for years.
In 2008, police tried searching Alissa's room. They also requested the note as evidence. Michael wouldn't let them into his home, and instead of the real thing, gave detectives a photo copy.
Courtesy Sarah Turney Sarah and Alissa as children
Ever since, "Michael Turney has refused to cooperate with the investigation," according to police documents provided to PEOPLE, which further notes he "is the only member of the Turney family not to cooperate" with the investigation. To date, he has never sat for a "formal" interview with authorities.
That same year, a search warrant was sought and issued, and when police got inside Michael's home, they recovered dozens of homemade bombs and a 97-page manifesto detailing his plot to commit mass murder at the local headquarters of the electrical workers union. In 2010, he pleaded guilty to possession of unregistered destructive devices, and was released after serving seven years in prison.
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The 2008 search also revealed Michael had installed surveillance cameras throughout his home. He also recorded all calls to and from his home, according to police. Alissa had $1,860 in her bank account when she vanished, and never accessed it in the six months after her disappearance, when bank records indicate Michael had the funds transferred to his account.
Courtesy Sarah Turney An undated photo of Alissa and Michael
"Michael Turney has exhibited an apparent obsession with his step-daughter, Alissa," police wrote in 2008. "He admitted to conducting surveillance on her at work, using binoculars to spy on her." Relatives also told detectives Michael treated Alissa differently than his other children, giving her "inordinate attention." Cops also seized a number of homemade contracts between Alissa and Michael. One contract, from 1999, declares Michael never molested her.
While Michael has repeatedly asserted his innocence, including during a 20/20 interview with John Quiñones in 2008, he's expected to stand trial sometime in 2021.
Sarah says she's "optimistic" justice is coming for Alissa, and, with the second season of her podcast, she plans to highlight other cold cases. "I feel this is my calling now," says Sarah.