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SACRAMENTO, Calif. (KTXL) — Authorities have identified the body of a woman found 39 years ago on the side of Interstate 5 in Sacramento.

Thanks to DNA technology, the Sacramento County Sheriff’s Office has identified the 26-year-old woman as Lily Ann Prendergast.

“Horrible. I couldn’t figure out why I couldn’t find her,” said William Prendergast on Tuesday.

Prendergast said the mystery of what happened to his little sister has haunted him for 40 years.

“She used to call me on my birthday all the time,” William Prendergast recalled. “That stopped.”

In late 1980, Lily Ann Prendergast left her father’s home in Dallas, Texas.

William Prendergast said his 26-year-old sister, who took off hitchhiking, was a free spirit and was always traveling.

“That’s the last anybody had heard from her,” he told FOX40.

More than a thousand miles away, detectives with the Sacramento County Sheriff’s Office were dealing with a mystery of their own. A woman’s decomposed body was found on the side of Interstate 5 in October of 1981.

“It’s a case that’s kept me up at night,” said Sacramento County Sheriff’s Deputy Paige Kneeland. “You know, I wake up thinking about it sometimes.”

Kneeland said they tried facial reconstruction, sketches and DNA.

“They used clay — just like you’d see in the movies — based off of a picture of the skull,” Kneeland said. “And in 2005, we got that profile entered into the missing person’s DNA program.”

In 2019, William Prendergast decided to submit his DNA to the FBI.

“One hundred percent match. It was her,” William Prendergast said.

While he was relieved to finally know where his sister was, he said his mind quickly shifted to the person responsible for her death.

“Hatred. I wanted to immediately find out who he was and track him down,” William Prendergast said.

The death is being investigated as a homicide.

“If anyone met her, if anyone knew her, please give us a call,” Kneeland requested.

“Who and why? Why would they do such a thing?” William Prendergast asked.

Investigators urge anyone who may have come across Lily Ann Prendergast to give the Sacramento County Sheriff’s Office a call.