The year was 1986 and Lindsey Wade, 11, already knew about fear, evil and bad people.

It had happened to Michella Welch, 12, first. She was taken in broad daylight while riding her bike near Puget Park, a small park on the north end of Tacoma, Washington. Search dogs found Michella that night. She had been sexually assaulted and murdered.

Five months later, Jennifer Bastian, 13, was riding her bike at Point Defiance Park, just a few miles away. When she didn’t come home, her family called the police. Hundreds of people searched. Three weeks later, they found her body hidden in the underbrush, with her bicycle nearby.

Jenni Bastian and Michella Welch. Courtesy of Pattie Bastian and Barbara Leonard

Watch the full story on NBC News' "Dateline" this Friday at 10pm ET/9pm CT.

“I just remember that it was really scary to me as a young girl,” said Wade, who spent her childhood in Tacoma. “There would be certain times where if I was out riding my bike or if I was walking, you know, through — a little trail to shortcut to get to school or something like that — you know — it would be something that I would think about.”

Det. Gene Miller was a patrol officer back then with the Tacoma Police Department.

“It's a painful thing. It's a painful thing as a parent. It's a painful thing as a community member,” Miller said. “And I think that it does one of two things to you: it's either gonna eat you up or it's gonna motivate you to find the bad guy.”

Police followed hundreds of leads, but no arrest was made. Detectives continued to search for the killer, but the case went cold.

In high school, Wade read “The Stranger Beside Me” by Ann Rule, the true-crime book about serial killer Ted Bundy — who was also from Tacoma.

Detective Lindsey Wade Courtesy of Detective Lindsey Wade

“I was fascinated by the book and terrified at the same time,” Wade said. “I just decided that that's what I wanted to do for a living. I wanted to catch people like him."

Gene Miller did, too. That is why he became a detective and in 2009, started the cold case unit at the Tacoma PD.

“Things have changed dramatically in how cases are investigated,” he said. “There's so much more that can be done.”

When Miller met Wade, he saw something in her. She was coming up through the ranks at the Tacoma PD from patrol to narcotics, then eventually made detective.

“She's got that grit, that determination, that commitment, that ability to fight through whatever challenges there are and just make things happen,” he said.

By the time Wade became a cold case detective, there were dozens of binders of police reports related to the cases of both Michella and Jenni. From those reports, she made a list of all the men mentioned in those pages. Every man connected to the case. There were 2,300 names.

“My working theory at that time was this guy has gotta be somebody who's been convicted of a sex crime or another murder,” Wade said. “And somehow, he slipped through the cracks.”

She had a suspect’s DNA from Michella’s body, but none from Jenni’s case because it was too badly decomposed by the time they found her. And the DNA found on Michella’s body didn’t match anyone in the FBI’s national database.

In 2013, at the advice of an expert panel at the National Center for Missing & Exploited Children, Miller and Wade decided to test Jenni’s swimsuit.

“I think the assumption prior to that was that her swimsuit must have been removed prior to any sexual assault occurring, because it was found down around her ankle at the time that she was recovered,” Wade said.

When they sent it out for testing, the lab discovered male DNA.

Detective Gene Miller Courtesy of Detective Gene Miller

“That was a big turning point in the investigation,” Wade said.

When they compared the DNA from Jenni’s swimsuit to the DNA from the suspect in Michella’s death — it was different.

“It was a shocker,” Wade said.

For almost 30 years, they had been searching for one person who was believed to have killed both Michella and Jenni. But that was wrong.

There wasn’t one killer. There were two.

For the next five years, Wade continued to investigate. She utilized advanced DNA techniques, forensic genealogy and collected DNA samples from 160 men. Her persistence paid off when Robert Washburn was arrested in May 2018 for Jenni's murder. He pleaded guilty to first degree murder in January and was sentenced to almost 27 years in prison.

In May 2018, Gary Hartman was arrested for Michella’s murder. He has pleaded not guilty and is awaiting his trial.

"Gary Hartman is facing the most awful of allegations. It is very easy to presume someone guilty based solely on these charges. At all times, he has maintained his innocence," his lawyer, Bryan Hershman, told NBC News.

"The defense is pursuing various investigative leads. There are profound challenges in starting an investigation 32 years after a tragedy occurs. Mr. Hartman has lived an honorable life. For decades, he has been a licensed registered nurse in Washington State."

"He held a very important job at Western State Hospital. He is a family man. In view of the horrific nature of these allegations, Mr. Hartman’s cry of innocence, and the very profound challenges in investigating a three-decade-old killing, I ask that the public accord Mr. Hartman the presumption of innocence to which he is entitled. Let the judicial system decide this case."

In April 2018, Wade retired from the Tacoma PD. She’s now working as an investigator with the Washington Attorney General’s Office. Her focus is assisting law enforcement with processing and investigating untested sexual assault kits.

She is also pushing to get a bill passed in Washington that would expand DNA collection. It’s called Jennifer and Michella’s Law.