Suffolk police believe the remains of Shannan Gilbert, a missing Jersey City, N.J., woman last seen 19 months ago, are in a marshy area near Oak Beach.

After multiple searches using dive teams, K-9 units and military surveillance, police Wednesday found a cellphone believed to be Gilbert's. On Tuesday, a pocketbook containing Gilbert's ID, and separately, a pair of tattered jeans and ballet-slipper shoes, were uncovered.

"She's in there someplace, and we're going to do everything we can to find her," Suffolk Police Commissioner Richard Dormer said Wednesday.

The search will continue today.

Gilbert, 24, who advertised online as a prostitute, went missing after leaving a client's house in Oak Beach on May 1, 2010. Her disappearance spawned an unprecedented search along Ocean Parkway that uncovered 10 sets of remains -- eight women, one infant, and a man dressed in woman's clothing -- between Dec. 11, 2010, and April 11.

Police said recently they believed all were the victims of one serial killer and all of the victims were somehow related to the sex industry.

A pipe installed by Suffolk County vector control helped drain the area where the pocketbook and other items were found. The pipe "drastically increased our visibility in there and assisted us in being able to find things," said Insp. Stuart Cameron, commanding officer of the Suffolk police special patrol bureau.

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Dormer said, "It's our belief that her body or parts of her remains may be in that location."

Mari Cox Gilbert, of upstate Ellenville, is skeptical that police found items belonging to her daughter, the eldest of four sisters.

"I am still doubtful," she said. "I need to see the items for myself to believe it."

Shannan Gilbert, police said, may have just wandered into a mucky area and drowned.

"The detectives who have been closest to the case have always believed it wasn't murder," Suffolk County Police Chief of Detectives Dominick Varrone said yesterday.

The items recovered and their locations are indicative of a person running wildly into a marshy area in the dark, Varrone said.

Mari Gilbert does not believe her daughter drowned. "She was running. She was screaming for help," she said. "Somebody was chasing her. Someone put her in that water. It was no accident."

Under gloomy skies and at-times driving rain, Suffolk police used brush cutters to clean narrow paths, a brush hog to create trails, and an amphibious vehicle to move into dense areas and look down into the brush, reeds and other growth.

Searchers included 10 people with cutters, 12 K-9 officers, six emergency services personnel and three members of the crime scene unit. Metal detectors were also used, and a crime scene van and medical examiner's vehicle were on hand for Wednesday's search.

Conditions were ideal for searching because the tide was low and poison ivy and ticks are less of a concern now than during the warmer months, said Cameron.

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But even then, officers were working in waist-deep conditions and at times became stuck.

"We've had officers abruptly drop to their waists or deeper in that muck, essentially like quicksand, and they've been unable to extricate themselves without the assistance of fellow officers," Cameron said.

Thursday, Shannan Gilbert's family plans a trip to Oak Beach, to see the place where she went missing.

Suffolk Police Officer John Mallia, while taking his K-9 Blue on a training exercise, found the remains of one woman on Dec. 11. Two days later, the bodies of three other women were found -- all wrapped in burlap. They were soon identified as Megan Waterman, from Scarborough, Maine; Maureen Brainard-Barnes of Norwich, Conn.; Amber Costello of West Babylon; and Melissa Barthelemy of the Bronx.

And Tuesday, Mallia was back searching. It was he who found the pocketbook.