How a discarded napkin was the key in solving the 32-year-old mystery of a girl's abduction.

On March 26, 1986, Michella Welch was abducted from a vast, thickly-forested park in Tacoma, Washington.

The 12-year-old girl had been playing in Point Defiance Park with her two young sisters at the time of her disappearance.

After an extensive search of the area, a police dog located her body in a ravine later that night.

She had been sexually assaulted and then killed with a knife.

Despite finding her remains within hours of her disappearance, and the gathering of crucial evidence at the crime scene, the case quickly turned cold.

Now, 32 years later, police have arrested a suspect. Gary Hartman, 66, will face court on Monday, charged with the rape and murder of the Tacoma preteen.

Hartman’s arrest has been three decades in the making.

As Sky News reports, in 2006 police scientists were able reconstruct a DNA imprint from the items collected at the crime scene.

Unfortunately, at the time, the reconstructed DNA could not be linked to any known criminal in the United States.

This year, with improvements in genetic genealogy, police were able to track down the killer.

Th technology, and its massive databases and archives, led police to two brothers who had lived in the area at the time of Michella’s murder.

The brothers were placed under surveillance. An undercover investigator sat at the next table as one of the brothers, Gary Hartman, ate dinner at a restaurant one night.

When Hartman left, the investigator swooped in and grabbed his used napkin.

The team then analysed the DNA on the napkin and it was match for the DNA left at the crime scene 32 years ago.

“Genetic genealogy uses DNA technology to identify subjects by matching the unknown profile to a family member,” Tacoma police chief, Don Ramsdell, explained at a press conference on Friday.

“Traditional genealogy is then used to build a family tree from publicly available websites.”

According to Oxygen, at the time of the murder, Hartman had been living within walking distance to the park’s entrance.

After the murder he stayed in the area, married, and became a registered nurse.

As The Spokesman-Review reports, Hartman’s arrest came as a surprise to his close-knit community.

“Here’s an example of when you think you kind of know people and you don’t,” a neighbour said.

“He had four or five collectible cars that he and his wife would always drive around. Very cordial and very sweet older people. The neighbors thought he was greater than sliced bread.”