Sources tell Eyewitness News it began as an argument over rent and ended with a tenant dead and a landlord in police custody.
The incident happened on Shepherd Avenue in East New York just after 1 p.m. Sunday.
Authorities say the fight may have been over as little as $100.
Police say the landlord, 63-year-old Felipe Estevez, armed with a gun, was trying to collect back rent from a mother of two who was locked in her apartment with the children.
That's when a neighbor, 51-year-old Luis Martinez, tried to come to the woman's aid and put himself between her door and the landlord.
After a confrontation, cops say the landlord opened fire, and once Martinez was down, he reportedly fired additional shots into his head and neck.
Martinez was pronounced dead at the scene.
After the shooting, police say Estevez broke down the door, pointed the gun at the woman pulled the trigger, but the gun jammed. That's when the woman's 12-year-old son jumped out a window and ran to the police precinct.
Estevez, now facing a murder charge, is claming self defense.
Neighbor Anna Maria Montes Deoca is beside herself with grief, after witnessesing part of the incident. She rents a basement apartment in the building, and she hear everything as it happened.
"Felipe was bathed in blood, it made me nervous," she said in Spanish. "It looked like they'd had a fight before he murdered the other man. He had a bat in his hand...I saw Luis on the floor. I pulled up his shirt. He was already not breathing."
Montes Deoca saw the boy jump out the window before he went to the police station.