In the end, it was a discarded piece of chewing gum, casually spit on the ground in 2021, that was the key to solving the cold-case murder of a college student that had confounded the authorities in Oregon for more than four decades.
Robert Arthur Plympton had been under police surveillance since the authorities determined that year that he was a “likely contributor” to a DNA profile developed from swabs taken from the body of Barbara Mae Tucker, who was 19 when she was murdered on the Mount Hood Community College campus in 1980.
On Friday, Mr. Plympton, 60, was found guilty of murdering Ms. Tucker after a three-week bench trial in Portland, Ore. According to The Oregonian, which reported on the investigation and Mr. Plympton’s conviction, it was the oldest cold-case homicide in Gresham, Ore., east of Portland.
On the night of Jan. 15, 1980, Ms. Tucker was expected at a class at the college, where she was studying business. She never arrived.
Students on their way to class the next morning found her “partially clad” body on a shrub-covered slope near a campus parking lot, The Oregonian reported at the time. There were signs that Ms. Tucker had been sexually assaulted, and that she had struggled with her assailant.
For decades, the authorities were unable to identify a suspect or make an arrest.
The first step toward a breakthrough in the case came in 2000, when vaginal swabs that had been taken during Ms. Tucker’s autopsy were sent to the Oregon State Police Crime Lab for analysis. Lab technicians were then able to develop a DNA profile from the swabs.
In 2021, Parabon NanoLabs, a Virginia company whose services include DNA-based forensics, identified Mr. Plympton as “a likely contributor to the unknown DNA profile developed in 2000,” the Multnomah County district attorney’s office said in a statement. It was not clear how the DNA connection was made; the district attorney’s office did not immediately respond to a request for comment on Tuesday.
Detectives with the Gresham Police Department found that Mr. Plympton was living in Troutdale, Ore., east of Portland and northeast of Gresham, and began surreptitiously watching him, prosecutors said.
When investigators saw Mr. Plympton spit a piece of chewing gum onto the ground, they collected it and submitted it to a state police crime lab, prosecutors said.
“The lab determined the DNA profile developed from the chewing gum matched the DNA profile developed from Ms. Tucker’s vaginal swabs,” the district attorney’s office said.
Mr. Plympton was arrested on June 8, 2021, as he was driving away from the Troutdale home he shared with his wife and son, The Oregonian reported.
He had a criminal record, including a conviction for second-degree kidnapping in Multnomah County in 1985, according to the Oregon Department of Corrections.
Mr. Plympton was 16 when Ms. Tucker was murdered. Witnesses reported seeing her with a man the night she was killed, and several people reported seeing her run into the street waving her arms, perhaps trying to flag someone down for help, The Oregonian reported.
Kirsten Snowden, the Multnomah County chief deputy district attorney, said at the trial that there was no evidence that Ms. Tucker and Mr. Plympton had known each other, according to The Oregonian.
Mr. Plympton’s lawyer, Stephen Houze, said at the trial that there was “unmistakable, unavoidable reasonable doubt” about who killed Ms. Tucker, according to The Oregonian. He said that witnesses had described the man who was seen with Ms. Tucker — who was nearly six feet tall — as being about her height or taller, but Mr. Plympton was closer to 5-foot-8. He also said investigators never tested Ms. Tucker’s clothing for DNA evidence.
“We will appeal, and we are confident that his convictions will be overturned,” Mr. Houze and his law partner Jacob Houze said in a statement on Tuesday.
Judge Amy Baggio of Multnomah County Circuit Court found Mr. Plympton guilty of one count of first-degree murder, and four counts of “different theories of murder in the second degree,” according to the district attorney’s office.
“To be clear, this court has zero doubt whatsoever that Robert Plympton beat Barbara Tucker in her head and face until she died,” she said at the trial. “He did.”
Judge Baggio did not find Mr. Plympton guilty of sexual assault, saying that prosecutors had failed to prove beyond a reasonable doubt that he had assaulted Ms. Tucker while she was still alive, The Oregonian reported.
Mr. Plympton is scheduled to be sentenced on June 21. Based on his age at the time of Ms. Tucker’s death, he faces a life sentence with the possibility of parole after 30 years on the first-degree murder charge.
Two members of Ms. Tucker’s family wept and embraced after the verdict was announced, according to The Oregonian. Ms. Tucker’s older sister, Alice Juan, said in a statement on Tuesday that her family was “thrilled that this was finally solved.”
“I thought it might not be as the years went on, but Barbie was a special little girl,” she said. Her little sister, she added, “was bright, bubbly, caring, all of those things.”
Kitty Bennett contributed research.