A Mamaroneck woman accused of killing her 4-year-old son pleaded not guilty Tuesday to one count of second-degree murder.

A Westchester County grand jury indicted Manuela Morgado, 46, earlier this month on one count of second-degree murder. She entered her plea Tuesday morning at an arraignment before Judge Barry Warhit in County Court in White Plains. She will appear again in court Jan. 10.

In a case that stunned the region, Morgado was arrested Oct. 1 after police, alerted by a relative's call, arrived at her Boston Post Road home to find her semiconscious and her son, Jason "Jake" Reish, dead.

Police said they believe Morgado -- who was in a bitter custody battle with the boy's father, Dr. Timothy Reish -- may have used helium gas in an attempt to suffocate Jason on Sept. 30 or Oct. 1 and then tried to commit suicide with a drug overdose.

Morgado recently had purchased a mask, tube and helium gas online, a law enforcement source previously told Newsday.

"Manuela Morgado took the life of her own innocent four-year-old son in a horribly misguided attempt to make sure the child's father would not have anything more to do with their lives," Westchester District Attorney Janet DiFiore said in a statement after the arraignment. "The fact that the defendant over an extended period of time, methodically planned, purchased the materials used and carried out the crime, made the ultimate outcome all the more tragic."

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In October, Morgado appeared in Mamaroneck Village Court, where protection orders were authorized for Jason's two stepbrothers -- Nick, 11, and Jack, 13 -- a spokesman for the Westchester County district attorney's office said. Nick and Jack are children that Reish had with his wife, Erin.

Reish, 43, and Morgado were not married but had a relationship when the doctor was separated from his wife.

Morgado is being held without bail at Westchester County Jail in Valhalla.

Assistant District Attorney Christine Hatfield is prosecuting the case. Morgado is represented by Legal Aid attorney Saad Siddiqui. A message left for Siddiqui was not immediately returned Tuesday.