Friends and relatives of the victims rushed to the hospital to sit vigil at the little boy's bedside.
9-year-old Jamar Henderson was in critical condition Wednesday afternoon after he was nearly beaten to death with a hammer.
His 45-year-old mother, Jacintha, lies in another room after being slashed across the face with a knife.
Police say it happened in the family's brownstone in Bedford-Stuyvesant, where their eldest son, Jonathan went on a rampage at 6 in the morning.
"Officers responded and arrested this individual. I'm not certain where he is in the process, whether he is in for psychiatric evaluation or what, but he was arrested this morning," Police Commissioner Ray Kelly said.
Investigators say it began with an argument, but between whom-and over what-is still not clear.
"He acts-out, he's not himself all the time," said Jahlessa Tonge, a neighbor.
Neighbors told Eyewitness News Jonathan Henderson's behavior seemed erratic at times, but never violent.
The suspect was being held by another relative when police arrived.
He was taken to Wyckoff Hospital for psychiatric observation.
He was described as emotionally disturbed and bi-polar.
The hammer and knife were recovered at the scene. The suspect was later arrested.
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