The night Shannan Gilbert disappeared, she called the police to tell them her life was in danger.

The operator kept her on the line for half an hour, but Shannan, a sex worker on a home visit, was either too scared or too drugged to say where she was: a housing development on Oak Beach, a remote stretch of Long Island.

Exactly a year ago today, a police dog searching for her remains found the bodies of four other women wrapped in burlap sacks, spaced a few metres apart, in the dunes along Ocean Parkway behind nearby Gilgo Beach. It was the first sign that a serial murderer – the Long Island killer – was on the loose. A fifth victim was discovered in March, then five more in April. Although officially she is still missing, not dead, Shannan's last call was treated as a vital piece of evidence in the search for a prolific serial killer.

On Wednesday, police said they had found a purse, containing her ID card, plus the ballet shoes and jeans she was wearing when she ran into the dark, terrified that someone was trying to kill her. Her last client, Joseph Brewer, and her driver, Michael Pak, have been ruled out as potential suspects.

Suffolk County Police Commissioner Richard Dormer says there is still no proof she's part of the serial killer case, saying that she could have been " running that night from Brewer's home into the thick brush, where there's water and sinkholes, tangles and bushes. It's horrendous terrain, so it would be real easy for her to lose her footing and drown".

Gilbert's mother, Mari, has travelled to Gilgo Beach to be near the search. "I am still doubtful. I still hold out hope," she says. "I don't buy the drowning theory. She was scared of the water." The mothers and sisters of the serial killer's victims are a tragic sorority, searching for clues, counselling each other and keeping the case in the public eye. They are planning a vigil, on the sand, to mark the anniversary of the first grisly discoveries.

Of the five identified victims, all were prostitutes who advertised on Craigslist, the online classified website. Jessica Taylor was the first reported missing, in July 2003. Her head and hands were found by Ocean Parkway, but her body was buried an hour's drive away, in Manorville. DNA from another torso at the site matched two severed legs discovered in nearby Fire Island in 1996.

Police now believe one man was responsible for at least 10 murders. The remaining five victims – three women, one toddler girl, believed to be the daughter of one of the women, plus a young East Asian man, dressed in women's clothes – remain unidentified.

"Asphyxiation appears to have been the cause of death for all nine females," says Dormer. "The male is possibly a rage killing. Our theory is that the other women that we haven't been able to identify were working in the sex business and that's why they aren't listed as missing on the national database. Prostitutes go missing, unfortunately, and because they're often estranged from their families, nobody notices that they're gone."

FBI profilers believe the killer is a local who knows the area well. He is an "organised killer" who plans his crimes meticulously, is probably married and owns an expensive truck. Following the disappearance of Melissa Barthelemy in July 2009, a man used her phone to call her family, to taunt them and eventually to boast that he had killed her. Because these calls all lasted less than three minutes, making them impossible to trace, there has been speculation that the killer has a background in law enforcement and knows how to avoid detection.

Dormer, an Irishman whose second cousin Jimmy Delaney won two league titles with Celtic, is sceptical of this theory. He believes television detective shows create unrealistic expectations. He says: "I call it 'the CSI syndrome' – people watch these shows, there's always DNA, someone always confesses. Well, in real life, this is not the case." He is due to retire at the end of the year but is confident that his colleagues will finish the job.

"What homicide detective would not like to catch a serial killer," he says. "It would be a feather in my hat, but I understand that this is a slow-moving investigation."

In the past, families of the victims have been critical of the police, but for now, they are publicly supporting the investigation. "It's heart-wrenching to keep talking about but we have to show that the victims were real people," says Melissa Cann, whose sister, Maureen disappeared in July 2009. "We formed bonds that aren't from blood, but from tragedy."

But after a week of intensive searching, with dogs and divers, Shannan Gilbert has not been found, and even if she is, police will be no closer to identifying the serial killer, or preventing him from striking again.