The police were again circling the Franz Bakery Outlet in the spring of 2017, hungry for information, and now she knew why.

The shop, a branch of a commercial outfit supplying baked goods to customers across the region, was on an empty two-lane road north of Bellingham, Washington. The investigators had come once before in 2013, asking about a delivery truck driver named Timothy Bass. Strangely, they had wanted to know details of his daily routes decades ago.

Now, four years later, a female co-worker – who has not been publicly named – asked why. They told her: Bass was a suspect in the 1989 rape and murder of 18-year-old Mandy Stavik, one of the region’s most infamous unsolved crimes.

The co-worker had a daughter herself. If it was her girl who had been viciously killed, she wondered, wouldn't she want someone to do whatever they could?

“After I asked if it was that case and who it was, I felt a basic human moral obligation to help,” the woman would later testify, according to the Bellingham Herald.

She handed over the information the police wanted even though her bosses had previously declined to assist. Later, a detective called back, asking if Bass ever ate food at work. She understood.

“What do you need, and I’ll get it?” she reportedly said.

On August 10, 2017, the co-worker watched Bass toss out a plastic cup and a Coke can. She secretly grabbed both, put them in a Franz Bakery bag and handed them over to police. When the results came back from the state crime lab, Bass’s DNA from the soda and cup were a 1 in 11 quadrillion match to DNA recovered from the dead young woman, prosecutors say. On December 12, 2017, Bass was arrested and charged with first-degree murder.

But if this anonymous woman’s freelance forensic work, pushed by a desire to do right, was the means of closing a 28-year-old cold case, it was also what threatened the prosecution’s case. For the last year, Bass’s lawyers have tussled with the state in court, arguing that the DNA match to Ms Stavik was the result of an illegal search.

“Put simply, law enforcement officers cannot use private citizens to obtain evidence without a search warrant where a search warrant would otherwise be required,” the defence wrote in one motion filed in August, the Herald reported. “This search was done for a singular reason: to assist law enforcement.”

Mandy Stavik was a bright and gregarious young woman. Growing up in Washington’s Whatcom County, she was reportedly a star student and athlete at Mount Baker High School – varsity basketball and softball; flute, clarinet and saxophone in the school band; an honour student who spoke Japanese and was fluent in sign language.

“She was a high achiever,” a former teacher told the Associated Press last December. “She wanted to do well in everything she did. She had it all going for her; she had a bright future ahead of her.”

In the fall of 1989, Ms Stavik was a freshman at Central Washington University. She had reportedly gone to college intending to be a commercial pilot, but abandoned the idea after she realised she would rather be “looking out the window rather than [at] the instrument panel,” a former teacher would say later.

She returned to her hometown of Acme, Washington, for Thanksgiving break. On the Friday after the holiday, Ms Stavik left for a jog around 2:30 p.m. wearing a light-coloured sweatshirt, green sweatpants and blue running shoes, according to the Herald. Kyra, her German shepherd, bolted off with her.

Hours later, the dog returned home, but Ms Stavik did not.

Authorities began searching. Two days later, they discovered a pair of abandoned green sweatpants in the woods. The next day, November 27, Ms Stavik was discovered floating in the Nooksack River, three and a half miles from her family home. She was naked except for socks and her blue sneakers.

Authorities determined Mandy Stavik had drowned, but there was also an injury to the back of her head that indicated she may have hit the water unconscious, the Herald reported. Semen was found on her body and preserved. The investigation kicked into high-gear.

“We’re still tracing down 7,859 leads,” a Whatcom County sheriff’s detective told the local press. “We’re going back over some of the old leads and trying to weed out those that seem far-fetched.”

But authorities hit roadblocks and dead ends. They drove ahead as the years piled up into a decade, then two decades. According to a news conference held after Bass’s arrest by Whatcom County Sheriff Bill Elfo, eventually his investigators tried to shake loose names from the community in the 2010s.

“The community was repeatedly asked for assistance solving this case,” Elfo said last December, according to a video. “Information was received pertaining to literally hundreds of potential suspects, and leads were followed up locally, in other states, and foreign countries, including in Asia.” One of the names now on the list was Bass.

He was not originally a suspect in 1989, according to Elfo. But when investigators approached him about providing a voluntary DNA sample to exclude him from the crime, he refused, Whatcom County Prosecuting Attorney David McEachran told a court at Bass’s initial appearance on December 13, 2017.

The lead detective on the case “also discovered Mr. Bass lived on Strand Road near Mandy Stavik’s home when she disappeared in 1989,” McEachran explained to the court. “At the high school, he started attending the girls’s basketball games, and watching her play basketball. When Mandy jogged with her dog, the route that she would take would take her past Timothy Bass’s residence.”

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All the factors contributed to the police’s interest in Bass as a suspect by 2013. But investigators apparently did not have enough evidence for a search warrant to compel a DNA sample from Bass. And his employer, Franz Bakery, would not voluntarily allow investigators to swab for a sample without a warrant or subpoena. The same went for information on his routes.

Then, in 2017, detectives returned to the bakery and encountered the woman who helped them land the case by providing the information about his 1989 deliveries and the Coke can.

But following Bass’s arrest, his attorneys attempted to have the DNA evidence thrown out because they argue the search violated his Fourth Amendment rights.

“I thought that it was kind of obvious that they had used an intermediary to collect evidence against Bass in an area where they were previously prohibited from getting evidence themselves, which violated a fundamental privacy interest,” Bass’s attorney told the Herald.

Last August, a judge ruled the evidence was admissible because the woman came up with the idea on her own. The decision was appealed by the defence. Last Friday, Bass learned his final appeal over the evidence had been denied.

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“I’m disappointed at the appellate court’s decision,” his attorney told the Herald. “Now the focus is only on preparing for trial and showing that Mr. Bass had nothing to do with this case.”

Bass’s trial is scheduled to start in April. He remains in custody.

The defendant does not have a criminal record, but has had previous run-ins with the court system.

In August 2010, Bass’s wife reportedly filed a domestic violence protection order against her husband. She alleged in court filings that he was both physically and verbally abusive to her and their three children.