Suffolk detectives have made progress investigating the slayings of as many as 10 people whose remains were discarded in the Gilgo Beach area, including four women who authorities believe died at the hand of the same killer, Police Commissioner Richard Dormer told a national news program.

"An arrest is not imminent, but we're making progress," Dormer told "48 Hours Mystery" in an hourlong special about the Gilgo case, which aired Tuesday night on CBS. Newsday viewed an advance copy.

But Dormer said it's still unclear what happened to Shannan Gilbert, 24, the Jersey City woman whose May 2010 disappearance in Oak Beach launched a search that later turned up 10 sets of human remains along the barrier island east of Jones Beach.

"I don't know," Dormer said of how Gilbert disappeared. "That's the $64,000 question."

The "48 Hours Mystery" special profiled each of the four possible serial-killer victims: Megan Waterman, 22, of Scarborough, Maine; Melissa Barthelemy, 24, of Buffalo; Maureen Brainard-Barnes, 25, of Norwich, Conn.; and Amber Costello, 27, of West Babylon. As did Gilbert, all four women worked as prostitutes.

The show also explored Gilbert's activities before she vanished on May 1, 2010, running screaming into Oak Beach after a visit there with a man who answered her online prostitution ad. It examines the accounts of her driver, Michael Pak, and her client, Joseph Brewer, who both told "48 Hours Mystery" that they did not harm her. Police have said neither is a suspect.

The actions of an Oak Beach doctor, Peter Hackett, a former director of Suffolk County emergency services, are also scrutinized in the broadcast.

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Gilbert's mother, Mari Gilbert, of upstate Ellenville, told Newsday she received a phone call from Hackett shortly after her daughter disappeared. She said Hackett told her he runs a halfway house for runaway children, that her daughter visited him on May 1, 2010, and that she left that day with her driver.

Mari Gilbert said Hackett asked her to have Shannan call him. "I said, 'Fine, I will.' Then he hung up," Gilbert said Sunday.

Until Sunday, Hackett had denied to Newsday and others that he called Mari Gilbert. He first confirmed the call to "48 Hours Mystery," but he denied Mari Gilbert's account of what was said.

Hackett Sunday told Newsday he made the call on May 6 at the request of Shannan Gilbert's boyfriend Alex Diaz, who visited Oak Beach to search for her. Hackett said he spoke with Mari Gilbert for less than four minutes and only offered his help as a board member of the Oak Beach homeowners association.

"I had nothing to do with anything occurring the night Shannan went missing," Hackett told Newsday Sunday. "I never saw her that night; she never came to my house, I never offered her assistance."

Hackett said Suffolk police searched his home with cadaver-sniffing dogs earlier this year, but found nothing. Police have said he is not a suspect.

With Andrew Strickler