NEW YORK — Samuel Joseph, 15, was just getting out of school Feb. 22 when his older brother got into a fistfight with another young Brooklyn man. What brought them to blows remains unknown, and the confrontation did not last long.

The altercation was of the sort that often leaves nothing but bruises and hurt pride. Instead, by the following morning, Joseph was dead, shot execution-style in front of his sister at their apartment in Flatbush. His brother’s opponent in the fight, Martial Amilcar, 20, was on the run.

Amilcar was arrested Monday night in Queens and was being questioned by Brooklyn detectives at the 70th Precinct station house.

The police said that on Feb. 22, Amilcar showed up in the lobby of the Joseph family’s apartment building and shot Joseph in the face, head and chest. Earlier that day, Amilcar and a second man had been searching the neighborhood for Joseph’s older brother. The police did not say which of 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 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 Amilcar confronted him in the lobby.

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

Police on Tuesday night were still looking for Amilcar’s associate, who they said had waited outside Joseph’s apartment building while he was killed.

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

Two hours later, Amilcar, wearing a hood, was seen outside of Joseph’s apartment building on Flatbush Avenue. He approached Joseph’s sister and asked if the older brother with whom he had fought was home.

Amilcar, who also lives in Flatbush, can then be seen on security camera footage entering the lobby of the family’s apartment building. He confronted Joseph there while his associate waited outside. In a matter of seconds, Amilcar pulled out a pistol and shot Joseph. Afterward, he was taken to a nearby hospital, where he died.

On Tuesday evening, a young woman who identified herself as Joseph’s sister said she expects more arrests to be made.

“I’m hoping they get all of them and put them away somewhere,” the woman said while standing inside the hallway of the family’s building on Flatbush Avenue, a few steps from a memorial to her brother that was outside. She declined to give her name, citing concerns for the family’s safety.

Monalisa Ferrari, a community liaison who is assisting with wake and funeral arrangements for Joseph and is serving as a family spokeswoman, said the victim’s mother, Raymonde Samuels, has been particularly hard-hit by her son’s death. Ferrari said the family took some solace from news of an arrest.

“I would say they’re relieved,” Ferrari said, “because having a criminal like this on the loose is worrisome.”

Chief William Aubry of the Detective Bureau said Tuesday that it remained unclear whether Amilcar intended to kill Joseph or had mistaken him for his older brother.

Murders across the city have spiked to 51 in the first two months of 2019 compared to 34 over the same period last year, a trend that has concerned officials in the department and at City Hall.

On Monday, Mayor Bill de Blasio and police commissioner James P. O’Neill said at their monthly news conference on crime statistics that it was too early to tell if the increase was a fluke, or a harbinger of a new trend.

This article originally appeared in The New York Times.