A discarded napkin has helped catch a man who allegedly raped and murdered a 12-year-old girl 32 years ago.

Gary Hartman, 66, is due to appear in court on Monday charged with rape and murder in the case, which shocked the city of Tacoma in Washington state for decades.

The mystery began on 26 March 1986, when Michella Welch disappeared while playing with her two young sisters in a park.

A police dog found her remains that evening in a ravine but the investigation got nowhere and was eventually filed as an unresolved "cold case".

More than three decades later, advances in the research field of DNA – the human genetic code – helped resurrect the investigation.

In 2006, police scientists reconstructed a DNA imprint from items collected at the crime scene, although no link was found to any known criminal in the US.

Twelve years later, the case progressed thanks to genetic genealogy, with its massive databases and archives, including from people researching their family history using DNA testing kits from specialist websites.

Tacoma police chief Don Ramsdell said: "Genetic genealogy uses DNA technology to identify subjects by matching the unknown profile to a family member.

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

The technology led police to two brothers, whose age and place of residence in 1986 eventually made them suspects.

The pair were placed under surveillance as police waited for a chance to verify their genetic profiles.

It is alleged Gary Hartman recently ate at a restaurant, oblivious to an undercover investigator sitting at the next table.

When Hartman left the investigator collected his used napkin and sent it to a lab for analysis.

Police say they discovered the DNA matched that found on Michella Welch's body.

Mr Ramsdell said: "This case does truly represent the crossroads of good old-fashioned police work combined with improvements in technology."

Pierce County prosecutor Mark Lindquist said it served as a warning to criminals.

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He said: "Today we are at a point where, if you are a criminal and you left your DNA at the scene, you might as well turn yourself in now. We will catch you."

In April, genetic geneology led to the arrest in California of a man suspected of being the "Golden State Killer" – blamed for 12 murders, some 50 rapes and scores of robberies in the 1970s and 1980s.

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