Rex Heuermann, the alleged Gilgo Beach serial murderer, kept a large room in his basement shut and was nervous when an interior designer assigned to measure his property tried to enter, warning her it was full of guns.

Katherine Shepherd said in an interview that she thought Heuermann’s response was strange at the time and now wonders what else he might have been concealing in the 12′ x 15′ room.

‘I didn’t understand why he was being so weird about it, and now I’m thinking “What was he hiding?” ‘ Shepherd said.

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Shepherd, 47, spent three hours in 2005 evaluating Heuermann’s Massapequa Park, New York house for a remodeling project. This was just one of many close encounters she had with Heuermann that have kept her up at night since his arrest last week, she said. Shepherd worked with Heuermann for five years.

In an interview, she discussed how she had gotten along well and worked with the 59-year-old architect, who is shown mixing with the interior designers and other professionals in newly released, exclusive images.

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She admitted that Heuermann even took her to a shooting range in the Bronx once and taught her the use of a 9mm weapon.

Heuermann once took her to the hospital after she fell on some ice, then accompanied her back to her Manhattan apartment where he gave her medication.

Shepherd collaborated occasionally with Heuermann from 2002 to 2007, sharing an office with him in Manhattan for two of those years while he was the owner of an architectural firm.

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She would frequently accompany him on job site visits as a self-employed interior designer. He was intelligent and largely amiable at the time, if socially uncomfortable, she said. She now compares him to a real-life Dexter, the fictional serial killer from Showtime.