In an extraordinary twist of fate, the key DNA evidence that helped to convict David Frediani came from the very body of work on which the scientist had been engaged at the time of her death.

A jury in Vista, California, decided that Frediani, 46, a financial analyst from San Francisco, murdered Helena Greenwood, who was 34, in the garden of her home in southern California in 1985. He will be sentenced on March 9 and will probably be jailed for life without parole.

Ms Greenwood had moved to the United States in 1977 with her husband, Roger Franklin, having graduated from Sheffield University and completed her doctorate in biochemistry at the University of London.

The couple spent a year travelling across the country in a camper van and then settled in Atherton in Silicon Valley - the area where she worked as a research analyst specialising in DNA.

In 1984, Frediani, then aged 30, broke into their home. Although Frediani, from nearby San Mateo, was an intelligent man who had held down a number of jobs in the financial sector, he also carried out burglaries in wealthy areas.

He held Ms Greenwood, who was alone, at gunpoint for a number of hours and sexually assaulted her. During long conversations, she persuaded him not to kill her and convinced him the she would not contact the police.

Frediani was not masked and Greenwood was able to provide police with enough information from what he had told her for them to find him and charge him with burglary and sexual assault.

At an initial court hearing, Frediani, who faced a 20-year sentence for the assault and death threats, was granted bail. His trial was scheduled for the summer of 1985.

In the meantime, Ms Greenwood was approached by the research company Gen-Probe of San Diego and offered a post of vice-president and a chance to continue her DNA research, working on the reading of gene sequences.

The couple moved south and found a house outside the city in Del Mar and continued to take safety precautions as Frediani was still at large.

Then in August 1985, three week's before Frediani's trial was due to start, Ms Greenwood did not arrive for work and her colleagues contacted her husband. He rushed home to find her strangled and beaten body in their garden. Within hours, police tracked down Frediani who was immediately the prime suspect. Credit card transactions placed him in the San Diego area just before the murder but he claimed to have had other reasons for being there and said that he had been 500 miles away in San Francisco at the time of the killing.

Police had no evidence that linked Frediani to the body. He stood trial for the sexual assault, was convicted on the basis of the statements that Ms Greenwood had made earlier. He was jailed for six years.

But although detectives were certain that Frediani was the killer, they were unable to get sufficient evidence to bring the case to court. Frediani was released after serving only three years.

In 1999, San Diego police reopened around 300 unsolved murders to see whether new scientific techniques would help them to find the perpetrators.

There had been skin particles found under Ms Greenwood's fingernails which were re-examined. Experts tried to see whether the advances in the DNA techniques on which Ms Greenwood herself had been working were sufficient to create a match with Frediani. They were.

Frediani was found, arrested and charged with murder. Again he pleaded not guilty, sticking to his story of being in San Francisco at the time of the killing. This jury did not believe him. He was convicted and has indicated that he will appeal.

"Without advances in DNA," said prosecutor Valerie Summers, "this murder might have gone unprosecuted. It's been a long time coming to justice".

Ms Greenwood's father, Sidney, from Lymington, Hampshire, who is suffering from cancer, welcomed the news and congratulated the police on their persistence. His wife had died shortly before the murder.