Police hunting the serial killer of five prostitutes in Sufffolk have drawn up a list of fewer than 50 suspects whose movements and activities are being actively monitored, sources have told The Times.

Detectives investigating the killings say some of the suspects were clients of the dead prostitutes.

At a press conference at Ipswich police headquarters Detective Chief Inspector Stewart Gull, who is heading the inquiry, insisted that his detectives were making "good progress". He said his message to the people of Ipswich was: "Don’t panic."

He said: "We have got a range of individuals who have been suggested to us. Some are local and some are not. We are determined to find the person or persons responsible."

He confirmed that the fifth woman to be murdered was Annette Nicholls, 29, whose body was found close to that of Paula Clennell, 24, at Levington, near Ipswich. In common with two of the other victims Ms Nicholls was naked except for her jewellery.

A post-mortem examination carried out late last night failed to reveal how Ms Nicholls died but tests are continuing. The others murdered women were strangled or asphyxiated.

Ipswich police confirmed that they were treating her death as murder and that they are linking it to the other four murders.

Mr Gull said: "The launch of a fifth murder inquiry in such a short space of time is unprecedented for Suffolk Constabulary."

In an appeal for information that could catch the killer, the father of the youngest victim said that drugs had taken his daughter into a "secret world" of prostitution.

Jim Duell said that neither he nor Tania's mother, Kerry Nichol, had known that the 19-year-old was working as a prostitute.

Seated beside Kerry Nichol, who Tania lived with, he said: "Tania was a lovely daughter – she was a caring, loving, sensitive girl who would never hurt anyone. Unfortunately, drugs took her away into her own secret world – a world that neither of us were aware of.

"Tania has been taken by someone who needs to be found. We ask for anyone who knows this person or persons to come forward and contact the police.

"We would like to thank all the people who have offered help so far but ask if anyone has any information, however small, please tell us - even if they come forward anonymously."

Tania Nicol, a former Ipswich hotel worker, was reported as missing by her mother on November 1 after she had not seen her for 48 hours. Ms Nicol's body was found just over a month later, on December 8. Hers was the second of the five bodies to be recovered.

Mr Duell also made a point of making his own personal statement to the families of the other women.

He said: "They cannot take away our memories, they cannot take away our love, our fortitude, our courage.

"Grieve for our daughters but not unnecessarily ... they would want to see us getting on with our lives and not going round with our heads bowed down.

"There is a time for sadness and a time for gladness. A gladness that they belonged to us and a gladness that they are now at peace."

Kerry Nichol, who remained silent throughout their joint appeal, seemed overawed by the number of television cameras and photographers, taking deep breaths as if trying not to cry.

The killer is believed to have knocked out his victims with drugs before strangling or suffocating them.

The Forensic Science Service said that it had made its priority to run toxicology tests on the five women’s bodies, to discover whether any drug or chemical had been administered to knock them unconscious.

Suffolk police have requested extensive tests on the bodies of Gemma Adams, 25, and Ms Nicol, the first of the killer’s victims. Post-mortem examinations on their bodies, which were immersed in water for some time, have proved inconclusive.

The Times understands that the same tests will be requested on the remains of Anneli Alderton, 24, Ms Clennell and Ms Nicholls.

All the women, who worked as street prostitutes in Ipswich, were addicted to heroin, crack or both. Scientists will be looking for evidence that they were given an overdose of a Class A drug or some other substance that left them incapable.

The tests have been ordered partly because detectives are puzzled that none of the bodies of the murdered women, whose bodies were left naked in rural locations on the edge of Ipswich, showed signs of a struggle.

Other than evidence of strangulation on the bodies of Ms Alderton and Ms Clennell, there are no signs of serious injury to the women. None of the dead was subjected to a violent sexual assault.

Mr Gull said today that he could not yet confirm whether the women had been drugged and warned that the tests will take some weeks and may be complicated because of the women's drug use.

He said five separate inquiry teams had been set up to investigate each murder.

"Each is staffed with specialist staff and headed by a fully-trained senior detective," he said.

"All these resources are committed solely to catching those responsible for the murder of these five vulnerable women."

He said more than 250 police officers were working on the inquiry and Suffolk police had received assistance from 26 police forces around the country.

Detectives have received 1,800 calls from the public in the last 24 hours, on top of the 5,500 already received.

Mr Gull described the response as fantastic. He appealed for information from anyone who might have seen Ms Nicholls between December 5 and December 12 when her body was found.

He reiterated his appeal for information about anyone who may have used two lay-bys on the Old Felixstowe Road near where the bodies of Ms Nicholls and Ms Clennell were found in Levington.

He said Ms Nicholls was wearing dark-grey patterned leggings, calf-length boots, a black top with a low neckline and a dark-coloured bomber jacket.

"We are extremely keen to trace Annette’s clothing, as well as the clothing of the other women," he said.

Ms Nicholls, who had no fixed address, was last seen in Norwich Road in Ipswich on December 5.

He said that there were a number of people of interest to the police but did not give further details.

The inquest into the death of Ms Adams has already opened. The other inquests are expected to be opened early next week.

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