A suburban New York man drowned his mother in the family's backyard pool after she argued with him because he wasn't taking his medication, prosecutors said.

Denis Cullen was arraigned Thursday in Suffolk County on a charge of second-degree murder.

Newsday reports (http://nwsdy.li/2bF2m5U ) that the 23-year-old Cullen pleaded not guilty to killing his 63-year-old mother, Elizabeth Cullen. Bail was set at $5 million cash or $8 million bond.

Assistant District Attorney Robert Biancavilla said Cullen confessed to drowning his mother in the family's swimming pool on Long Island on Wednesday.

"This is a son who killed his mother, and the way he killed her, the way he describes, how it was done, is chilling," Biancavilla said.

Biancavilla said the two had argued over Cullen not taking his medication. The prosecutor said the argument escalated to the point where she poked him and Cullen put his mother in a headlock, walked her to the deep end of the pool and held her until she stopped struggling.

"He said she struggled violently and he was surprised a woman of her stature could struggle as much as she did," Biancavilla said.

After the arraignment, defense attorney Steve Fondulis described his client's demeanor as "calm."

"I have to determine if he had a psychiatric background. I have to determine if he's under a doctor's care," Fondulis told Newsday.

Elizabeth Cullen was the daughter of Maj. Gen. George William Casey, who was killed in a 1970 helicopter crash in Vietnam. Her brother, George William Casey Jr., is a retired general who last served as Chief of Staff of the U.S. Army.

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Information from: Newsday, http://www.newsday.com