A 911 operator heard the teen yelling that he had a gun, police said.
“The circumstances of how it occurred at this point is under investigation and you can rest assured that we will take this very seriously,” said New York Mayor Michael Bloomberg.
Officers received the 911 call from the teenager’s mother about 7pm on Monday, police spokesman Paul Browne said. The 911 operator could hear the teen yelling in the background, claiming he had a gun, Mr Browne said.
When officers arrived at the Brooklyn apartment building, they could see 18-year-old Khiel Coppin pacing inside the first-floor apartment. His mother was outside.
The boy’s mother had attempted to have him hospitalised earlier in the day, Mr Browne said. He said the teen had a history of mental illness.
The teen began screaming from a window at his mother and officers before climbing out of the window and heading toward the officers holding a black object in his hand, police said.
The officers ordered him to stop, Mr Browne said. When the teen refused and kept approaching them, they opened fire, he said.
Police said it was not immediately known how many of the 20 bullets struck Mr Coppin, who was pronounced dead at a hospital.
Investigators later discovered the object the teen was holding was a hairbrush, Mr Browne said.
The case nonetheless evoked painful memories of previous police shootings: the November 2006, 50-bullet barrage that killed the unarmed Sean Bell on his wedding day, and the 1999 killing of unarmed African immigrant Amadou Diallo, who was hit by 19 of the 41 shots fired by police in the Bronx.