Warning: This story has graphic details that were released with the family’s permission.

TAMPA, Fla. (WFLA) — Investigators in Florida said they have solved the 1986 cold case murder of a nurse after nearly 37 years, thanks to an “illicit” affair by a relative of the suspected killer.

On Oct. 27, 1986, the mother of Teresa Lee Scalf, Betty Scalf, found her daughter dead in her Lakeland home.

The victim’s family said her mother had to break into the residence with a credit card after the hospital told her Teresa didn’t show up for work.

“I think she was killed around 3:30 in the afternoon, and by the time I got there at 8 or 9 o’clock at night, all the evidence had been washed away by the rain,” Betty Scalf said.

The Polk County Sheriff’s Office said Teresa Scalf’s throat had been slit and she had “defensive wounds” on her hands. Detective Matt Newbold said there was no sign of forced entry before the victim’s mother got there.

“He cut her head almost off,” Sheriff Grady Judd said.

After years of investigation, detectives identified the murder suspect as Donald Douglas, who lived behind Scalf at the time of her murder. They said the murder was sexually motivated.

The 29-year-old nurse, who worked at what is now Lakeland Regional Health Medical Center, lived alone with her 8-year-old son at her apartment. However, the child was at his grandmother’s home at the time, deputies said.

Scalf’s family described her as a loving, kind person.

“She used to cry when she had to stick babies for blood, and she cried when the teenagers came in [after] car wrecks,” Teresa’s sister Lynne Scalf said. “She liked working ICU because she provided comfort.”

Her sister Pam Shade, who is also a nurse, said she got her family into health care. Sadly, Teresa had only become a registered nurse one month before her murder, according to her sister.

“Our family didn’t deserve this,” Shade said.

Detectives found two blood samples, one of them belonging to Scalf. But without the proper technology, there was no way to identify who murdered her.

“When this occurred in 1986…there was no DNA genealogy,” Judd said.

The sheriff said in the 2000s, the Polk County Sheriff’s Office put the unknown blood sample into the Combined DNA Index System (CODIS), but there was no result.

“Detectives talked to Donald at the time of the event as they were doing neighborhood canvases, but there was nothing at all out of the ordinary,” he said.

Then, in 2022, the sheriff’s office teamed up with Othram Inc., a DNA genealogy company that works with law enforcement agencies. Othram analyzed the blood sample and narrowed the scope of the search to distant relatives of the suspect, the sheriff’s office said in a press release.

Investigators looked for a DNA match through genealogies and family histories, which eventually led them to the suspect’s third cousin who had a child outside her marriage in 1949, the Miami-Herald reported. The affair pointed them to a “new line of the family tree,” the newspaper explained.

From there, investigators were able to trace the suspect’s son.

“When we were able to work the genealogy up into the modern era, Matt met with Donald Douglas’ son and said, ‘We’d like a blood sample. We’re working a murder investigation,'” Judd said. “The son [of] Donald Douglas was totally cooperative and was in horrible shock. His dad, and here’s the key to it, had never ever ever been arrested. Ever.”

The sheriff said Douglas’ son never thought his father would ever murder anyone. However, his DNA did match the blood sample found at the scene.

“The results of a genetic analysis of the DNA confirmed that the blood found at the murder scene did indeed belong to Donald Douglas because the DNA profile indicated a 100% confidence of a parent/child biological relationship compared to Douglas’ son’s DNA profile,” a statement from the sheriff’s office reads. 

According to Lynne Scalf, Teresa did tell her sisters that she had a creepy neighbor who was “stalker-ish” but never described him.

“If you are getting a creepy vibe, don’t just tell your sisters this creepy neighbor is doing weird things,” she said. “Give a description. If we had had that description the sheriff just showed you, that would have narrowed it right down. Large man, scruffy, 30-ish? Think of what that would have done to the potential pool of suspects.”

Douglas was 33 at the time of the murder but died at the age of 54 of natural causes in 2008, the sheriff’s office said. Detectives said Douglas’ body was cremated.

Teresa Scalf’s family thanked law enforcement for their dedication to solving her murder.

“I’m 84 years old,” Betty Scalf said. “I lived to see this done. I think that’s why I lived so long.”