The police said that on Feb. 22 Mr. Amilcar showed up in the lobby of the Joseph family’s apartment building and shot Mr. Joseph in the face, head and chest. Earlier that day, Mr. Amilcar and a second man had been searching the neighborhood for Mr. Joseph’s older brother. The police did not say which of Mr. Joseph’s older brothers — he is one of eight children — was involved in the original fight.

The gruesome killing of the teenager shocked the city and rattled Mr. Joseph’s neighborhood, where his family of immigrants from Haiti had settled about a decade ago. The boy was a sophomore at the High School for Youth and Community Development at Erasmus. He had been leaving his apartment to get something to eat when Mr. Amilcar confronted him in the lobby.

Mr. Joseph was remembered by friends and business owners in the neighborhood as “a good kid” and a talented basketball player.

The murder was a sobering reminder of the city’s violent start to 2019, driven by a spike of homicides in Brooklyn — 24 so far in 2019, 14 more murders than during the same period last year.

On the day of the murder, the police said that a security camera recorded Mr. Amilcar and Mr. Joseph’s older brother getting into a fight around 3:45 p.m., though they declined to say where it took place.