The 33-year-old police officer, Aaron Hess, was a former Marine who for seven years had served on the tiny police force in Pleasantville, N.Y., the village where he had grown up. He had never fired his gun at anyone in the line of duty.

Brandon Cox, 20, was a college running back sitting next to his best friend from Easton, Mass., Danroy Henry Jr., in a car outside a Westchester County bar. Mr. Henry, known as D. J., was a football player, too, with no record of trouble, whose arms, which held the car’s steering wheel, were tattooed with the words “Family First.”

During 15 or so seconds early on the morning of Oct. 17, Officer Hess fired four rounds, his lawyer said, into the Nissan Altima with Mr. Cox and Mr. Henry inside, killing Mr. Henry, wounding Mr. Cox and igniting a mix of confusion and anger.

Officer Hess has told his lawyer that the shooting was justified and that the events began when he heard another officer yell for Mr. Henry to stop his car as he abruptly drove off from near the bar, Finnegan’s Grill in Thornwood.