New York authorities said they have identified at least 15 credible reports of the accused Gilgo Beach serial killer Rex Heuermann having contact with sex workers.

Suffolk County Sheriff Errol Toulon Jr. revealed to Newsday that since Heuermann’s arrest last year, they have interviewed 298 sex workers housed in correctional facilities in Suffolk and Nassau Counties. Toulon said they also plan to interview sex workers at Rikers later this month.

Last July, police arrested Heuermann in Manhattan for the 2009 and 2010 murders of three women at Gilgo Beach on Long Island. Heuermann is charged with the murders of Melissa Barthelemy, 24; Megan Waterman, 22; and Amber Lynn Costello, 27. The following January, he was charged with also killing 25-year-old Maureen Brainard-Barnes.

Barthelemy, Waterman Costello, and Brainard-Barnes were reportedly wrapped in burlap and buried next to each other on Gilgo Beach.

Last month, Suffolk County prosecutors charged Heuermann with murdering Jessica Taylor and Sandra Costilla.

Taylor vanished from a bus terminal in Manhattan in 2003. Days later, her torso was discovered in a wooded area about a mile from the edge of the Peconic River Sportsman’s Club. In 2011, her skull, hands, and forearm were reportedly found on Gilgo Beach — less than a mile from where the original four victims were located.

Costilla vanished in 1993, and her remains were found the same year in North Sea, Long Island. She is Heuermann’s earliest known victim, according to prosecutors who claimed that Heuermann accessed pornographic images which coincide with how Taylor and Costilla’s remains were found.

Heuermann has pleaded not guilty to all six slayings. He is due back in court on July 30.

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[Featured image: James Carbone/Newsday via AP, Pool]