Detectives set out to confirm that the socks were a pair, using photo-enhancing technology.

On Nov. 12, 1991, Theodore Dill Donahue decided it was time to clean out his apartment.

Earlier that day, a tersely written news item had appeared at the bottom of the 14th page of the metro edition of the Philadelphia Inquirer, noting that a woman's body had been found in the woods outside the city, where it had been abandoned weeks earlier. Authorities hadn't yet identified the woman as Denise Sharon Kulb, but Donahue, who had been dating the 27-year-old before her death, apparently felt it was time to get rid of the belongings that she had left behind before moving out. He showed up at Kulb's mother's house on the opposite side of Philadelphia later that day, handing over a pile of clothing.

But there was one thing that he didn't bring with him: a long, ribbed yellow sock.

"He decided to keep that, for some reason," Anthony Voci, the supervisor of the homicide unit for the Philadelphia District Attorney's Office, said at a Tuesday news conference.

Now that missing sock has emerged as a crucial piece of evidence, although it's not clear whether Donahue intended to keep it or simply forgot about it. On Tuesday, nearly 28 years after Kulb's body was found, the Philadelphia District Attorney's Office and Pennsylvania State Police announced that her former boyfriend had been charged with murder. Reopening the cold case had been complicated by a lack of DNA evidence, so detectives instead revisited clues gathered nearly three decades ago, including the yellow sock. That led them to Donahue, who had chosen "TedBundy1967"as his email handle, combining the serial killer's name with his own birth year, prosecutors said.

The 52-year-old, who is being held without bond, also faces charges of evidence tampering, abuse of a corpse, obstruction of justice and making false reports to police. His attorney, R. Emmett Madden, told the Philadelphia Inquirer that Donahue maintains he is innocent.

"He denies the charges, and we will dispute it in court," Madden said.

At the time of Kulb's death, she was described in the Inquirer as a "streets girl" whose wild, free-spirited nature led her to drop out of high school and, ultimately, into a devastating heroin addiction. As her drug problem worsened, she began working as a prostitute, lost two children to foster care, handed two others over to their grandparents and ran into trouble with the law.

A neighbor told the Inquirer that she was "always in trouble with drugs and alcohol and men," which worried her mother and sisters. But her older sister, Debra Kulb, who worked at a bar under the elevated train tracks in their working-class neighborhood, recalled how Denise had always walked in singing and dancing, making it impossible not to like her.

"She always wanted everybody to love her," she said. "She just looked for love in the wrong way."

It's unclear how the outgoing mother of four met Donahue, who has worked as a pizza delivery man in Philadelphia for the better part of three decades, according to authorities. Officials say that she moved into his apartment on a sloping street lined with rowhouses at the start of October 1991. To her family, it had seemed like things were looking up: Debra Kulb told the Inquirer that after Denise moved in with her boyfriend, she seemed to get her addiction under control, and they started letting her spend time with her oldest daughter on weekends again.

"You could see that her confidence was up," Kulb told the paper. "She was going to job interviews, she was glowing."

But she moved out of Donahue's apartment after only two weeks. Then Denise's family learned that she had returned to sex work. When they gathered for a funeral on Oct. 19, 1991, they angrily confronted her about it.

"I beat her up," Debra Kulb told the Inquirer. "That was the last time I ever saw her."

That same day, family members told police, Denise got into a fight with Donahue outside the bar where her sister worked. She had moved out of his apartment just days earlier, according to investigators, and it was the last time anyone would see her alive. No one reported her missing, and nearly a month passed before police got a call about an unsettling discovery roughly 30 miles southwest of Philadelphia near the Delaware state line.

That November, a resident of Chadds Ford Township, Pa., noticed that something seemed to be buried under the foliage near a secluded and undeveloped cul-de-sac. It was Denise Kulb's body, which was so badly decomposed that it took the coroner's office days to figure out who she was and to rule her death a homicide.

Donahue was "obviously a person of interest" from the start, Voci said Tuesday. A friend told police that the pizza delivery man, who was 24 at the time, had previously made alarming comments about how Kulb was "not coming back." Although Donahue denied any involvement, he did admit that his nickname was "Ted Bundy," and he repeatedly called police in the days after his interview to ask about the autopsy results, authorities said. He also offered to help out with the investigation, in what detectives described as a "nervous manner."

But police didn't feel that they had enough evidence to arrest the slain woman's former boyfriend. By the time the 10th anniversary of Kulb's death came in 2001, the case had gone cold. Authorities offered a $2,000 cash reward for any tips, but another decade passed with no arrests. Then, in 2015, the Pennsylvania State Police decided to revisit the investigation. When they returned to Donahue, they found that his story had changed.

When initially questioned by police, Donahue said that he had last seen Kulb on Oct. 18, 1991, one day before her family remembered the two of them fighting outside a bar. He claimed that after buying and using crack together, they had been robbed at knifepoint, and she had run off to get help. Interviewed again in 2015, Donahue took back that story, saying that he had actually last seen Kulb outside a bar that day. He "admitted he had lied" in his original telling, Voci said, but didn't admit to any involvement in her death.

For the next few years, police worked their way through Donahue's circle of acquaintances. They learned that, over the years, he had recounted details of Kulb's death that only a perpetrator or eyewitness would know, such as how she had been found face-down in the woods after being strangled. He also told a friend that she had been found dead at his apartment in Philadelphia, Voci said.

Then there were the yellow socks. When Kulb's body was discovered, she was clad in only a sweater, according to authorities. But other clothing was piled on top of her - two pairs of pants, a T-shirt, a jacket and one pale-yellow knee sock. While searching Donahue's apartment back in 1991, police found an identical sock, along with a job application with Kulb's name on it.

After reopening the case in 2015, detectives set out to confirm that the yellow socks were a pair, using photo-enhancing technology. They relied on assistance from Temple University's photography department, which "dropped everything" to help, Philadelphia District Attorney Larry Krasner said Tuesday. What they saw confirmed their theory: The two socks matched.

Authorities said Tuesday that the separated socks are "a key piece of evidence" bolstering their case against Donahue, but wouldn't speculate on why police hadn't felt that way when they made the discovery nearly three decades ago. "I can't speak for what the investigators understood or believed at that point and time," Voci said.

Asked if the long delay between Kulb's death and Donahue's arrest had harmed the case, the prosecutor replied that the opposite was true. The passage of time, he said, allowed investigators to see how Donahue's story had changed over the years.

"Sometimes cases get better with time, just like wine does," he said.