A boy, 5, was killed and his seven-year-old sister is in hospital after eating pizza, laced with poison, police allege.
NBC New York reported the children's father, from the Bronx, in New York, was being questioned over the tragedy, which occurred on Tuesday night at an apartment where he was staying with his step mother.
The man had recently split up with his wife and had shared custody of his two children.
Police told NBC he allegedly phoned his ex-wife about midnight to tell her he was going to kill himself.
A short time later, he locked himself in the bathroom with his son.
Police gained access to the bathroom and found the body of the lifeless boy. He was rushed to hospital but was pronounced dead on arrival. His sister was suffering severe vomiting and remains in hospital.
The Daily Mail said police were questioning the father who allegedly admitted he poisoned the children and himself "with something he slipped onto the pizza".