Joan Francisco was found strangled in her home in St John's Wood, north London, on Boxing Day 1994 with the flex of her floor cleaner wrapped around her neck. Her former boyfriend Anthony Diedrick, a computer expert, was arrested at the time but protested his innocence and was not charged.
It was only four years later that a tiny amount of blood "that would not have survived a single wash" was found on her pink T-shirt and linked to him, Nigel Sweeney, prosecuting, told the Old Bailey yesterday. He said that there was only a one in 170 million chance of the DNA sample matching anyone else.
The court heard that the 27-year-old doctor had not had any "meaningful" contact with Mr Diedrick since her days at medical school. Yet the blood was found on the T-shirt she was wearing when she died.
Mr Diedrick, the prosecutor said, had developed a deep obsession with his ex-girlfriend. He had bombarded her and her family with calls, spied on her and rigged up her home telephone to monitor calls.
"The murder was the terrible culmination of the defendant's obsession with Joan Francisco, who wanted nothing more to do with him," Mr Sweeney said. "Joan Francisco ended their relationship in late 1988 or early 1989, about six years before the murder."
The couple had dated for about a year when the doctor was a student living with her mother in Acton, west London. Although Mr Diedrick had allegedly had affairs during their friendship "he could not accept Joan Francisco's rejection of him", said Mr Sweeney. "He become obsessed with the idea that he was in love with her and could get her back. She made it clear at all stages that she wanted nothing more to do with him."
For two years there was virtually no contact between the two. However, Mr Sweeney told the jury: "In the autumn of 1994... for no remotely sensible reason, this defendant rekindled his obsession that Joan Francisco was the love of his life and that if he could talk to her he could get her back." Mr Diedrick talked endlessly about her, wrote letters and telephoned her mother to find out where she was living.
From early November 1994, he began spying on the doctor at her flat and attempting to record her telephone conversations with a home-made device, the court heard. Mr Diedrick was also making calls to the switchboard of the Royal Free Hospital, north London, where she was working. She "promptly put the phone down".
Dr Francisco had been due to leave for a holiday in California on Boxing Day. "There is evidence that he thought she was leaving permanently," Mr Sweeney told the court. "It represented his last chance for some time to confront her and thus begin the process of getting her back."
The doctor's body was found at 1pm that day after two calls from her mother went unanswered. "No one saw or heard what happened," said Mr Sweeney. "She was alone at the flat ... The defendant got in, there was a confrontation and he strangled her both manually and with the flex of a floor cleaner from the flat."
Mr Sweeney said: "He did not leave fingerprints but he did leave one tiny clue: a tiny amount of his blood on her T-shirt. It was a clue that was to be missed for nearly four years."
Mr Diedrick was arrested in January this year and samples of his blood were taken. They were found to match three small spots from the garment's neck. Mr Diedrick, 39, of Maida Vale, north London, denies murder.
The trial continues.