The young man accused of killing his mother in their Lloyd Harbor backyard earlier this month not only drowned her, but also strangled her and likely beat her, a Suffolk prosecutor said in court Tuesday.

Denis D. Cullen Jr., 23, pleaded not guilty to an indictment charging him with second-degree murder in the Aug. 17 death of Elizabeth Cullen, 63.

Prosecutors had said that Cullen got his mother in a headlock by the side of their pool, dragged her into the shallow end and walked her into the deep end, drowning her — but Assistant District Attorney Robert Biancavilla said there was more to it than that.

An autopsy showed she was asphyxiated not only by drowning but by neck compression, Biancavilla said. In addition, her scalp had several cuts and her body was “covered” in bruises, he said, suggesting a protracted struggle before Cullen got his mother in the headlock and pulled her into the pool.

“The evidence against Mr. Cullen, judge, is overwhelming, to say the least,” Biancavilla said to Suffolk County Court Judge Stephen Braslow in Riverhead.

Mother and son argued in the house about whether he was taking prescribed medication, and then Denis Cullen walked through the yard and sat by the pool, Biancavilla said. His mother followed him there and the argument intensified, he said.

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Cullen told detectives during a videotaped confession that he was surprised by how much a woman as small as his mother could struggle while she drowned, Biancavilla said in court.

After Elizabeth Cullen died, Biancavilla said her son put the body next to their pool house and covered it with a dinghy so no one would find her. Denis Cullen then went back in the house, showered and took his mother’s credit card and car keys, and drove to the Cold Spring Harbor train station, Biancavilla said.

Cullen met a sister in Manhattan and told her what had happened, and she called their father, who then called Lloyd Harbor village police, Biancavilla said.

No family members came to court Tuesday.

Without explanation, Braslow replaced Cullen’s assigned attorney, Steve Fondulis of Port Jefferson, with John Halverson of Patchogue shortly before the arraignment.

“I don’t believe arraignment is the place to try the case,” Halverson said in court.

He asked Braslow to order a psychiatric evaluation of his client and to put him on a suicide watch, and Braslow agreed.

After the arraignment, Halverson said a psychiatric defense is possible.

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Biancavilla, however, said that when Cullen was talking about the killing with detectives, “He knew exactly what he was doing.”