A 46-year-old crazed man killed his sister-in-law with a knife in her Sheepshead Bay home and then turned his rage on a child and two other women inside the building, authorities said.
The surviving victims were also related to the attacker, according to the NYPD.
They are his mother-in-law, sister-in-law and nephew.
"This is a shock to me. They're a nice family," neighbor Elaine Friedman told NBCNewYork.
Police said the 4-year-old boy was stabbed in the shoulder and is in stable condition at Lutheran Medical Center. The women were slashed in the chest and leg. Both are in stable condition, an NYPD spokesman said.
The suspect is under arrest and a knife was recovered at the scene, cops said. The dead woman was described as a 37-year-old who was stabbed multiple times.
The attack happened around 6 p.m., at 2260 East 18th Street.
Police believe a dispute over money may have led up to the crime.