When Michael Turney reported his 17-year-old stepdaughter Alissa Turney missing in May 2001, Phoenix police believed she was a rebellious runaway. There was a note in her room saying she was headed to California, and her friends and siblings said she was weary of her stepfather’s overly strict rules and couldn’t wait to turn 18 and be on her own.

Then, in 2006, the case went from a presumed runaway to a possible homicide. A convicted murderer in Florida, Thomas Hymer, claimed to be a serial killer and seemingly confessed to killing Alissa. Investigators found his claims of being a serial killer and the confession to be false, but it resulted in two Phoenix detectives taking a closer look at the teen’s disappearance.

It wasn’t long before they began to suspect something wasn’t adding up, and they turned their attention to Michael Turney as a possible suspect. Turney maintained he was innocent throughout the investigation. He was eventually tried and acquitted of Alissa's murder. With no remains ever found, Alissa’s disappearance continues to haunt her family more than two decades later.

A new “20/20,” airing Friday, Sept. 15, at 9 p.m. ET, delves into Alissa’s case, featuring new interviews with Michael Turney, the lead detectives, a forensic psychologist, Alissa’s friends and family and journalists who have covered the case. ABC News first covered the disappearance of Alissa Turney in 2009 when Michael Turney sat down for an interview with John Quiñones.

Watch the all-new "20/20" episode "Since You've Been Gone" airing Friday, Sept. 15, at 9 p.m. ET and streaming the next day on Hulu.

Alissa Turney was born in 1984 to Barbara Farner and Stephen Strahm. Her parents divorced when she was 3 years old, and her mom got remarried to Michael Turney. They blended their families – Michael Turney had three kids from a previous relationship, while Farner had Alissa and an older brother, both of whom were adopted by Turney. The couple also had a daughter together named Sarah.

Alissa’s mom died of cancer when she was only 9 years old, leaving Michael Turney to raise Alissa and Sarah by himself. By then, the other four children were grown and already out of the house.

Alissa seemed to be a typical 17-year-old who liked to have a good time. James, one of Alissa's older brothers, recalls his teenage sister as having an "incredible smile" and that she liked to read and draw and was “incredibly social.” James also claims that Alissa had an “unpleasant" and "tense" relationship with their father, who he says was strict and controlling at home. Friends of Alissa also said her stepfather was strict and protective and that she would often complain about him.

When asked in 2009 if he was a strict father, Michael agreed, saying he was "very strict" and claimed it was "more of a task for a parent to protect Alissa and keeping her in school."

Before Alissa went missing, there were signs she wanted to go to California. In 2000, she wrote a driver’s ed essay in which she calculated how long and how far it would take to drive to California. In 2001, she asked her boyfriend in a phone call recorded by her father and later released by investigators, "Would your parents let you to go to California with me?” Alissa’s older brother says Alissa asked to live with him before she disappeared, and a maternal aunt in California was also willing to take her in for the summer at the request of Michael Turney.

On May 17, 2001, it was Alissa Turney’s last day of her junior year of high school. Her boyfriend, Jon Laakman, said Alissa told him that day that her stepdad was picking her up from school to take her to lunch.

At lunch, Michael says their conversation turned into an argument when Alissa said she wanted to be able to stay out later at night with her friends.

“I told her, ‘That’s just not gonna happen with this. You know, as long as you’re under my roof, we’re gonna have to check in with daddy, ‘cause daddy’s a nervous wreck if you don’t,” Michael Turney told ABC News in 2009.

“[Alissa] said for the last year or so, that she was ready to leave home. She wanted to go live somewhere else. I didn’t think she was ready. I didn’t want her to. I wanted her to stay,” Turney said.

Upon realizing Alissa wasn’t home and learning of the note in her bedroom, Michael says his “heart just completely stopped.” He started calling around to try and find Alissa and eventually called police later that night to let them know his 17-year-old daughter had run away from home.

Phoenix police say they deal with thousands of similar reports every year.

A week after the disappearance, Michael got a phone call early in the morning and told ABC News that the voice he heard belonged to Alissa.

“The conversation was sort of scrambled. I said, ‘Is this you Alissa?’ And she said a few cuss words about, ‘Leave me alone,’ and then the phone went dead,” Michael said in 2009.

Michael says his home phone recording system was not turned on when the call came in. He eventually obtained a record of the phone call which showed it originated from a payphone in Riverside, California - about an hour from where Alissa's aunt lived. Hoping to discover information about her, Michael traveled to Riverside and handed out flyers with Alissa's picture, but he did not find anyone who had seen her. Michael also contacted the National Center for Missing and Exploited Children to register Alissa as a missing teen.

Then, after the 2006 false confession from Thomas Hymer, a man serving a life sentence in Florida for the murder of his travel companion, detectives William Andersen and Stuart Somershoe started noticing some red flags in Alissa’s case.

Alissa had around $1,800 in savings, but never used any of it, police said. "When people run away, they don’t leave a note to say where they’re going, because they don’t want to be found," Somershoe said. Alissa also left her cell phone behind, which investigators thought was unusual for a teenage girl.

Then detectives began interviewing Alissa’s family and friends, who shared some disturbing allegations about Michael Turney.

“The friends, when we ask, ‘Is there anything negative in her life?’ They continually point out her relationship with her stepfather,” Andersen told ABC News.

Alissa’s friends claimed in interviews with police that Alissa told them Michael had attempted to molest her, Andersen said.

Michael Turney strenuously denies the allegations of abuse and having anything to do with Alissa's disappearance, telling ABC News in 2009, “All I can say, until hell freezes over, I didn’t do a damn thing to my daughter.”

Michael Turney had Alissa sign a contract that gave “extensive detail about what she agreed to do or not do until she was 18,” Dr. Erin Nelson, a forensic psychologist who worked on the case, told ABC News.

“It included all kinds of specifics about sexual conduct,” Nelson said.

The contract also included rules for Alissa to follow about not drinking alcohol or using drugs.

Another document “was a statement indicating that she agreed [Michael Turney] had never physically or sexually abused her,” Nelson said.

“They were incredibly disturbing. And way outside the bounds of even a protective parent-child contract," Nelson said.

When asked in 2009 if the contracts were abnormal for a parent, Michael Turney said, “Oh, it depends on who you talk to. If it would have avoided anything happen to my daughter, yes. I don't think it was far beyond. At the time, I look back at it, because she ran away as a result of that? Yeah, ‘course I feel bad about it.”

The case took a surprising turn in December 2008, when police executed a search warrant at Michael Turney’s home. There they found more than two dozen improvised explosive devices, 19 firearms, two silencers and a document detailing what, policy say, seemed like a plan to attack an electrical workers’ union building in Phoenix. Detectives say the document also mentions Turney's belief that assassins were hired by the union to murder Alissa and dump her body in Desert Center, California.

When asked in 2009 about what he had in his home, Michael Turney told ABC News, "Firecrackers, a few things to make some noise, start a fire...I wanted attention brought to Alissa."

He denied there was an alleged plan to bomb a union building, saying it sounded "insane" and questioning why he would "murder a bunch of innocent people."

Police conducted an extensive and detailed search of the home that Alissa last lived in, but did not find evidence of a crime scene.

Michael Turney went on to plead guilty to unlawful possession of unregistered destructive devices, and the other charges were dismissed as part of his plea deal. He was sentenced to 10 years in prison and was released in 2017.

But the explosive discovery was just the beginning.

Alissa's sister, Sarah, who once believed her father had nothing to do with her sister's disappearance, changed her perspective and took to social media to tell her personal story. She urged the Maricopa County District Attorney to take action against her father. Although authorities had not found Alissa’s body and had no forensic evidence linking Turney to a murder, Michael Turney was arrested and charged with Alissa's murder in 2020.

Michael Turney was acquitted by a judge in July. After the prosecution rested their case, Turney’s defense motioned for the judge to dismiss the charges under an Arizona court statute known as Rule 20, which allows the court to enter a judgment of acquittal on any offense “if there is no substantial evidence to support a conviction.”

“There has not been a shred of physical evidence in this case. No body, no crime scene, no evidence to suggest that a murder took place,” Michael Turney's attorney Olivia Hicks argued in court.

Judge Sam Myers said, “The court is deciding whether a reasonable inference can be made that the defendant caused or the defendant engaged in conduct that resulted in Alissa’s death. The court is unable to make that finding.”

“The court has considered the evidence and finds that substantial evidence does not exist to warrant a conviction,” Myers said.

When asked about the acquittal, Michael Turney, who has proclaimed his innocence all along, told ABC News, "Did it exonerate me from what my children were thinking, the negative stuff that they were fed? No, that damage is done...It might someday recover itself, but that's only going to happen if we find Alissa."