A Lloyd Harbor man confessed in “chilling detail” to police that he dragged his mother to their backyard swimming pool and drowned her after they argued over him not taking his medication, a prosecutor said Thursday.

Denis D. Cullen Jr., 23, pleaded not guilty to a charge of second-degree murder in the death of Elizabeth Cullen, 63. Bail was set at $5 million cash or $8 million bond at his arraignment Thursday in First District Court in Central Islip.

“This is a son who killed his mother, and the way he killed her, the way he describes how it was done, is chilling,” Assistant District Attorney Robert Biancavilla said outside of court.

Biancavilla, deputy chief of the Suffolk district attorney office’s Homicide Bureau, said Denis Cullen confessed to Suffolk County police investigators over several hours after he turned himself into cops at the scene of the killing Wednesday evening on White Hill Road.

Cullen and his mother argued earlier in the morning on Wednesday over him not taking his medication and he later dragged her into the pool and drowned her, the prosecutor said.

The drowning took place about 10 a.m. Wednesday, according to court documents.

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“The argument escalated to the point where she had poked him,” Biancavilla said. “After she poked him, he put her in a headlock, walked her from the shallow end of the pool with her in the headlock to the deep end of the pool, all the while she was struggling. He said she struggled violently and he was surprised a woman of her stature could struggle as much as she did.”

Neither Biancavilla nor Cullen’s defense attorney would specify the type of medication Cullen was supposed to be taking.

Denis Cullen then carried her body out of the pool and placed it next to a shed in the backyard and covered her body with a dinghy, Biancavilla said.

Denis Cullen then went into the home where he lived with his mother, father and one of his three sisters, and took a shower, Biancavilla said.

He then took money from his mother’s wallet, a credit card and her car keys and drove to the nearby Long Island Rail Road station in Cold Spring Harbor and took the train into the city, Biancavilla said.

Cullen, who has no previous criminal record, met up with one of his sisters in the city and told her what happened and she called their father and he called the Lloyd Harbor police and asked officers to do a wellness check. The three then returned to the home and Cullen turned himself in to cops, Biancavilla said.

Police said they received the call to do a wellness check about 3:45 p.m. Wednesday and found Elizabeth Cullen’s body at about 4 p.m. Denis Cullen surrendered at 6:50 p.m., police said.

Defense attorney Steve Fondulis of Port Jefferson said he was assigned by the court to represent Cullen and spoke to him for about 10 minutes before court and said that his demeanor was “calm.”

“There’s a lot of work to be done,” Fondulis said. “I have to determine if he has a psychiatric background. I have to determine if he’s under a doctor’s care. And I have to look at a videotape.”

Suffolk County Police Commissioner Timothy Sini and Det. Lt. Kevin Beyrer give details at a news conference about the arrest of Denis D. Cullen Jr. in the death of his mother at their Lloyd Harbor home on Wednesday, Aug. 17, 2016. Photo Credit: Joseph D. Sullivan

Asked if he had shown any remorse, Fondulis said, “He hasn’t been convicted of anything, so he’s not sorry at this point; he hasn’t said ‘I’m sorry.’ ”

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Asked if Cullen has a history of drug abuse or mental illness, Fondulis said he had “notations of some kind of history, but I don’t know what it is yet.”

No family members were in court, lawyers said, and none could be reached for comment Thursday. Cullen is due back in court Tuesday.

A Lloyd Harbor police officer was stationed outside the Cullen family home Thursday and turned reporters away from White Hill Road, claiming it is a private road. Records show the $1.8 million home is on 2 acres in the affluent North Shore village that rarely sees any major crime.

One acquaintance of the Cullen family, who didn’t want to be identified, said in a phone interview that her son had attended elementary through high school with Denis Cullen, who was known by his middle name Dyson. She said neither she nor her son had seen Dyson since they graduated from Cold Spring Harbor High School.

Elizabeth Cullen's body was found in the in the backyard of her home on Aug. 17, 2016. Denis D. Cullen Jr. was arrested and charged with second-degree murder in her death, police said. Photo Credit: Google Maps

“It was a very, very nice family,” she said. “It’s just a tragedy.”