How DNA is helping to keep two inquiries alive

The battered body of Samantha Class was found, partially naked, washed up on the shore of the River Humber last October. The former prostitute from Hull had been beaten and then strangled to death before being tossed into the water.

Five years earlier, guests at a wedding reception in Hampshire discovered the body of 15-year-old Helen Gorrie on a woodland path. Helen had also been strangled and some of her clothes had been removed.

Now both cases may be solved thanks to breakthroughs in DNA-sampling technology.

In Ms Class's case, Humberside police are turning to mass DNA-screening of suspects. Detectives have drawn up a list of about 1,200 men who are believed to have paid for sex in Hull's red light district. They want to compare the men's DNA profiles with six different samples obtained from semen at the 29-year-old's home, where she would take her clients for sex.

The suspected punters have been given a few weeks to provide samples voluntarily, after which the police will give them no choice. At first the men, most of whose families will be unaware of their trips to Hull's streetwalkers, will be sent official letters. If this fails they face "a knock at the door", according to one senior investigating officer.

The use of mass screenings to identify and eliminate suspects is becoming increasingly popular. The technique has been used more than 60 times in the past three years, resulting in the capture of 21 people.

In the case of Helen Gorrie, the police are believed to have obtained DNA from material trapped under the teenager's broken nails or possibly clothing. This was made possible because of the improved methods of obtaining DNA from tiny amounts of living and dead cells. Last week three men were arrested and released on bail in connection with the murder, in the grounds of a community centre at Horndean. Helen had been asphyxiated. There was no evidence she had been sexually assaulted.

Detective Superintendent Graham Tarrant, who is heading the inquiry, said: "Forensic science has moved on in the years since Helen's murder. Advances might make it possible to find evidence today which could not be detected in 1992. It has also enabled us to re-examine other unsolved cases of murder and rape."

Jason Bennetto