It began on Thursday night with a group of teenagers smoking marijuana in the lobby of a Bronx building. It ended three or four minutes later with one of them critically injured by a fall from the roof of the six-story building, lying in a dark alleyway with a police officer cradling his head.

On Friday, the victim, Hakeem Kuta, 17, of the Bronx, was in critical condition at St. Barnabas Hospital.

Mayor Bill de Blasio’s concerns about marijuana arrests disproportionately affecting the lives of young black and Hispanic residents led New York City to alter its stance about the drug: Since November, those caught possessing less than 25 grams of marijuana have been given summonses instead of being charged with a misdemeanor.

Yet the debate about whether the use of marijuana is an indicator of criminality or disorder or just an annoyance persists. It did on Thursday, at 7:35 p.m., when a man walked from 2685 Valentine Avenue, flagged four probationary police officers who were just starting their shift and told them about “kids in the lobby of the building smoking marijuana,” said Stephen P. Davis, the Police Department’s chief spokesman.