20/20 examines the case of the Yosemite Park serial killer, who killed four women in 6 months

Cary Stayner made headlines in 1999 after he terrorized Yosemite National Park, murdering four women within six months.

The "20/20" documentary dives into Cary's past, the horror that he inflicted and features the first primetime TV interview with former FBI Agent Jeff Rinek and the first TV interview with former FBI Agent John Boles, who together took Cary's shocking confession. The documentary also includes an exclusive interview with Lenna, one of Cary's original intended victims he described in his confession, who is revealing herself for the first time since the murders to "20/20."

"20/20" airs on Friday, January 25, 9-11 p.m. ET on ABC.

"20/20" reports the chilling story about Cary's younger brother Steven, who was kidnapped as a child and escaped after seven years in captivity and how Steven's disappearance may have impacted Cary's behavior. It also includes exclusive, never-before-broadcast audio of Cary's confession and footage from the ABC News archives.

Steven was kidnapped from his home in Merced, California at age seven. While in captivity, he was abused by captor Kenneth Parnell. At age 14, Steven escaped with Timothy White, another victim of Parnell's. Upon his return home, Steven was hailed as a hero and his disappearance became the subject of books and a movie. He later married, had two children, and tragically died in a motorcycle accident at age 24.