Jayden Rivera, 19, was arrested Wednesday night on charges of murder, manslaughter and criminal possession of a weapon. He remains at Westchester Medical Center, where he is undergoing evaluation.
Detectives believe the teen was babysitting his 5-year-old half-brother Kayden Rivera Saturday night.
They believe he went on the deadly rampage after his father, 38-year-old Jonathan Rivera, and his father's girlfriend, 33-year-old Hanoi Peralta, returned home to their Mott Haven apartment.
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Police say the victims were stabbed several times and believe the teen also beat them with a pot to the point that it became bent.
Neighbors heard the screaming but the bodies weren't found until daybreak.
The teen then went to his mother's apartment in the Bronx. She called an ambulance later Sunday afternoon after he said he was hearing voices.
He was taken to Westchester Medical Center where police say the teen uttered that he had killed someone.
Detectives obtained a search warrant for his mother's apartment and found bloody clothing and a pair of sneakers that matched a bloody footprint inside the murder apartment.