Oxygen's latest true crime special is exploring a murder case that took over three decades to solve.

In 1984, 12-year-old Jonelle Matthews disappeared from her Greeley, Colo. home, and not even a nationwide missing child milk carton campaign—one of the first in U.S. history—proved successful in helping find her.

"After 20 years, I just felt like there was nothing I could do," Angela Hicks, who was married to Jonelle's murderer at the time of her disappearance, admits in E! News' exclusive sneak peek at The Girl on the Milk Carton. "I tried to do something and that didn't work. It was a lot of waiting and a lot of years."

Angela had previously suspected her ex-husband, true-crime enthusiast and former Idaho gubernatorial candidate Steve Pankely, was somehow involved in her disappearance. But Angela's efforts to bring justice were unsuccessful—until 35 years later when Jonelle's remains were discovered in 2019 just 20 miles from her home.

That's when Angela got a call from Robert Cash, who reopened the tragic cold case.

"I work as a person's crime detective here at GPD," Robert tells Angeles in the preview during an unexpected phone call. "One of the cold cases that I have is a very old one. Its's from 1984 and it involves the disappearance of Jonelle Matthews."

Angela is understandably shocked to hear Jonelle's suspected killer might finally pay for his crimes.

"All these years later, seriously now?!" she reacts in a confessional. "This is the longest I've ever waited for a call back."

As their call continues, Robert asks, "From what I understand you were married at one point to Steve Pankey, is that right?" to which Angela confirms, "That's correct. We were married for 23 and a half years and we've been divorced for 17 and a half years."

Angela would later reveal the crucial evidence that ultimately led to Steve's 2022 conviction for kidnapping and murdering Jonelle. He's currently serving life in prison with the possibility of parole. (Steve maintained his innocence throughout the trial.)

Go inside Angela's decades-long journey for justice when The Girl on the Milk Carton airs Sunday, Aug. 25, at 7 p.m. on Oxygen.

(E! and Oxygen are both part of the NBCUniversal family)