British police said Friday they have identified the fifth woman believed to be the victim of a serial killer targeting prostitutes, but were uncertain of the cause of her death.
The last of the five victims to be formally identified was Annette Nichols, 29, said Detective Chief Superintendent Stewart Gull of Suffolk Police.
"A post mortem last night failed to reveal a clear cause of death," Gull said.
Gull also confirmed to FOX News that his office is monitoring the investigation into the murders of four prostitutes in New Jersey whose bodies were found in a ditch outside Atlantic City last month.
"I am aware of those deaths, but my focus is on the five murders here in Suffolk," Gull said, adding that he is willing to get in touch with American agencies, but his investigation is not there, yet.
On Thursday, police confirmed the death of Paula Clennell, 24, who told reporters days before her death that she was afraid, but that she needed to carry on working to support her heroin habit.
Police said she died from "compression to the neck," which a pathologist said almost certainly means strangulation.
Gull made a fresh appeal for anyone who saw Nicholls, 29, between her disappearance on Dec. 5 and Dec. 12, when her body was found.
Gull said detectives were trying to trace her clothing — she was last seen wearing dark-grey patterned leggings, calf-length boots, a black top with a low neckline and a dark-colored bomber jacket.
Asked about potential suspects, Gull said: "There are a number of people of interest to us."
Gull announced that 1,800 calls from the public had been received on the special helpline in the past 24 hours. That is on top of the 5,500 already logged.
Sky News reported that police were looking into the possibility that the women were drugged as none of the women were believed to have put up a fight before they died.
Assistant Chief Constable Jacqui Cheer said earlier that police had spoken to a driver of a BMW and were trying to ascertain whether it was the man described by local prostitutes as having been in the area where one of the victims — Anneli Alderton — was last seen.
Police have linked the five cases, in which all the victims' bodies were found naked in rural areas, but separate teams are investigating each death, Gull said.
The bodies were discovered in a 10-day span, beginning Dec. 2 when 25-year-old Gemma Adams was found in a stream.
The body of Tania Nicol, 19, was found in a pond six day later.
On Dec. 10, the naked body of 24-year-old Alderton was spotted in the woods, after initially being mistaken for a discarded mannequin. The two final bodies were found close together on Tuesday.
The bodies of the four women identified by Atlantic City police as known prostitutes were found last month in a watery ditch behind a seedy motel outside Atlantic City. Each of the bodies was missing shoes and socks, and their heads were turned to face the gambling mecca, leading to speculation of a serial or ritual killer.
FOX News' Greg Palkot and Edward Barnes, and Sky News contributed to this report.