Dec. 15 (Bloomberg) -- U.K. police investigating the killing of prostitutes in the eastern English county of Suffolk confirmed the fifth woman in the case was murdered.

She was formally identified as Annette Nicholls, 29. Her body was found Dec. 12. Tests have failed to establish exactly how she died, Detective Chief Superintendent Stewart Gull said today in a televised news conference. A sixth woman has been reported missing, police said. No further details were given immediately.

``We are treating this as murder and are linking it to the other four murders,'' Gull said. ``There have been no arrests. There are a number of people of interest to us.''

If the same person carried out all five murders they would be the worst U.K. serial killer of prostitutes since Peter Sutcliffe, known as the Yorkshire Ripper. He was sentenced to life in prison in 1981 for killing 13 women, some of them prostitutes, in northern England between 1975 and 1980.

More than 250 police officers are now involved in the Suffolk manhunt, Gull said. Suffolk Police are receiving help from more than half of the 43 police forces in England and Wales. Detectives received 1,800 calls from the public over the last 24 hours following their appeals for information, Gull said.

The five bodies were found in rural locations close to a main road, the A14. All were drug users, Gull said. They were all naked, though three didn't have their jewelry removed.

Gemma Adams, 25, was found in a stream near Ipswich on Dec. 2. Tania Nicol, 19, was found two miles away in the same stream on Dec. 8, and Anneli Alderton was found in woods on Dec. 10. Nicholls and Paula Clennell, 24, were found Dec. 12.

`Secret World'

``Tania was a lovely daughter, a caring, loving, sensitive girl who would never hurt anyone,'' Nicol's father Jim Duell said at a televised press conference. ``Unfortunately, drugs took her away into her own secret world. Tania has been taken by somebody who needs to be found.''

None of the victims appeared to have been seriously sexually assaulted, nor were there significant signs of struggle, Gull said. Tests showed Alderton was strangled and Clennell died as a result of her neck being ``compressed,'' Gull said.

Prostitution is not illegal in the U.K., though many activities linked to it are outlawed, including soliciting in a public place and living off immoral earnings. The government has published proposals to allow mini-brothels of two or three prostitutes working together in a private house, though no decisions have been made on the plans.

To contact the reporters on this story: Nick Allen in London at nallen14@bloomberg.net.

Last Updated: December 15, 2006 12:34 EST