Spokane police say they have identified a man who raped and murdered a 12-year-old girl nearly 35 years ago, marking the first time a local cold case has been solved through the use of genetic genealogy.

Marsi Leah Belecz ran away from her home in the East Central Neighborhood on the night of Aug. 3, 1985, and her mutilated body was discovered two days later in a nearby towing yard. The medical examiner counted 29 stab wounds on her chest and two on her head. Her throat also had been cut.

Despite hundreds of interviews and other exhaustive police work, Marsi’s killer went unnamed for decades.

On Wednesday, police announced they have matched a DNA sample taken from Marsi’s body to Clayton Carl Giese, a Montana native with a limited criminal record who was 22 at the time of the killing.

Giese will not be arrested, however. He died in January 1989 – fewer than three years after the murder – after failing to navigate a curve and rolling his car off Appleway Boulevard near Interstate 90.

Investigators began focusing on Giese last summer after recruiting Parabon NanoLabs, a Virginia-based company that specializes in genetic genealogy.

Parabon submitted the DNA profile from a vaginal swab to a public database called GEDmatch, which is similar to consumer genealogy sites like Ancestry.com. After building a large portion of Giese’s family tree, Parabon returned a list of four distant relatives who might have been Marsi’s killer.

Police obtained DNA samples from two of those relatives, but neither was a match.

Detectives needed to test Giese’s DNA, so they obtained a search warrant and exhumed his body from Saltese Cemetery in Spokane Valley on March 5, said Sgt. Terry Preuninger, a department spokesman.

Afterward, the Washington State Patrol’s crime lab in Cheney compared the DNA sample from Marsi’s body with samples taken from Giese’s body.

William Culnane, a WSP forensic scientist, said that analysis confirmed Giese was the killer with a random-match probability of 1 in 1.1 nonillion. A nonillion is a 1 followed by 30 zeros.

It’s “a nonillion times more likely” the DNA at the crime scene came from Giese than from “anybody else in the population,” Culnane said.

“It’s a very significant number,” he said. “It’s about as high as we get as far as certainty goes.”

Genetic genealogy has been pioneered as a law enforcement tool in recent years to solve cold cases, beginning with the so-called Bear Brook murders in New Hampshire and the crimes of the Golden State Killer in California. But consumers’ privacy concerns have prompted companies like GEDmatch to repeatedly tweak their policies for working with law enforcement agencies.

Giese’s name didn’t appear in previous reports from the investigation into Marsi’s death, Spokane police Capt. Brad Arleth said during a news conference Wednesday. It’s unclear if Giese knew Marsi, or if he had been at the house party where she was last seen alive.

“He had a very minor criminal background here,” Arleth said. “The database shows one arrest for, I think, a marijuana possession charge just a few months before the car crash that killed him.”

Marsi was born on Oct. 28, 1972. At 12 years old, she lived with her parents and four sisters at 1718 E. Third Ave. She was in the sixth grade at nearby Grant Elementary School.

Police say she ran away from home at about 7 p.m. that Saturday in August 1985, wearing a lilac-colored dress and some new shoes she had just purchased with her allowance.

“It was the first time she bought a new outfit,” one of Marsi’s sisters, Donna Vanzant, told KHQ in an interview last year. “And she was proud of herself because we never got to really do stuff like that. We lived in a strict family.”

Vanzant said Marsi ran away because her father had made some hurtful comments about her outfit and appearance.

“She was hurt more than anything,” Vanzant said. “She gave my mom a hug and said ‘I love you,’ and that was the last I seen her.”

Their mother called police and reported Marsi as a runaway on the afternoon of Sunday, Aug. 4. Police say Marsi was last seen at about 10 that night at a party at 1125 E. Newark Ave.

Monday morning brought a horrific discovery: The owner of a towing yard at 811 E. Pacific Ave. found her body under a rusty boom truck. She was still wearing her lilac dress. At the time, Giese lived about 2 miles east of the crime scene at 3908 E. Second Ave.

At the junkyard, police found a leather sheath made for a 5-inch, double-edged knife manufactured by Gerber, but other evidence was scarce.

Police say Marsi’s body had “no apparent defensive wounds.” Giese, who stood 6 feet tall and weighed 200 pounds, easily could have overpowered the child, who was 5 feet tall and 92 pounds.

Detectives Jim Peterson and Mark Bennett were assigned to lead the investigation. Shortly after the killing, Bennett told The Spokesman-Review that Marsi’s assailant likely was a man motivated by anger at women and girls.

That same month, police from multiple agencies raided homeless camps along the railroad and the Spokane River, searching for anyone who might have seen or heard something relevant to the case. A boat and a helicopter were used to find the encampments. Authorities interviewed 115 people, mostly transients, in an area spanning 2 square miles between Division and Havana streets. A $2,000 reward was offered for information leading to an arrest.

Over the years, police interviewed a total of 257 people and identified 87 potential suspects, a dozen of whom were ruled out through DNA testing.

In 2002, some suspect DNA was entered into the FBI’s CODIS database, which contains the genetic profiles of millions of criminals and arrestees. Police say it was one of the first cases entered into the database by the WSP’s crime lab. The CODIS database has helped police solve numerous other cases, but it did not generate a match for Giese.

Detectives Mindy Connelly, Mark Burbridge and most recently Brian Hammond have taken on the role of lead investigator on Marsi’s case. The Spokane Police Department does not have a cold case unit, so it has relied on detectives from the major crimes unit, which focuses primarily on recent killings, arsons and other violent offenses.

Recently, Vanzant told KHQ a detective had contacted her to say her sister’s killer finally had been identified.

“I’m still in disbelief, but I’m happy at the same time,” Vanzant said. “I feel free, like a big weight has been lifted off of me.”