Crime scene investigators spent hours Wednesday collecting evidence inside 86 West 179th Street.
Cops responded around 1:30 a.m. to a call of an emotionally disturbed man.
Inside, 47-year-old Lorenzo Espinal's apartment, police made a horrible discovery.
A 5-year-old boy was found face down in a bathtub and a 7-year-old girl was violently ill.
Both children were rushed to St. Barnabas Hospital.
Stewart Espinal died on the way, and sources now tell Eyewitness News his sister had been poisoned.
The children lived in a nearby apartment building on Walton Avenue with their mother.
She was reportedly separated from Espinal, but he was still in the picture.
Neighbor Francisco Merrano said he got a phone call from the children's mother Wednesday morning.
"She said, 'The father killed my baby,'" Merrano said.
Meanwhile, in Brooklyn, a 4-year-old girl was discovered dead with head and chest wounds in her crib in East New York.
Her mother says she put Halii McCord to bed and then found her dead in the crib in the morning. She is being interviewed by detectives at the 75th Precinct.
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