Photocopied images of Mr. Polanco were taped to a metal pole. The crowd parted and cleared a path for Ms. Reyes. Someone handed her a black marker, which she held aloft and pressed against a photo of her son dressed in Army fatigues. “Mom loves you,” she wrote, adding balloonlike hearts as bookends on either side of those three words.

The events leading to the fatal shooting began about 5:15 a.m. on Thursday, the police said. They said that Mr. Polanco was driving erratically, switching lanes while speeding, and twice cutting off two police trucks carrying nine officers of the Emergency Service Unit, the parent unit of the Tactical Apprehension Team.

The officers, members of the apprehension team, had just executed a warrant in the Bronx and were headed to Brooklyn to execute another warrant, and were traveling eastbound on the parkway near La Guardia Airport, the police said.

Image Noel Polanco

Mr. Polanco had just left Ice NYC, where he worked part time in the hookah lounge, filling and serving tobacco water pipes. Though he was not working, he had gone to the club to give a bartender, Diane Deferrari, and Ms. Deferrari’s friend, Vanessa Rodriguez, an off-duty police officer, a ride home. All three lived near one another in Corona, Queens.

The two police trucks forced Mr. Polanco to stop after one truck went in front of his vehicle, a Honda, while the second one maneuvered behind it.

After the car stopped, along a median of the busy highway, two officers approached the car, a sergeant at the driver’s side and the detective at the passenger side, where the window was open, the police said.Ms. Deferrari later told the police that she had heard the officers order those inside the car to show their hands. In an interview, she said that Mr. Polanco had no time to comply and that, in that instant Detective Hamdy fired the shot. Ms. Deferrari said she believed the shooting was the result of a case of police road rage.