To many experts, Officer Wilson’s actions in the confrontation with Mr. Brown — as he described them to the grand jury — were within the bounds of standard police protocol. Officer Wilson testified to the grand jury that the two struggled over his service weapon while he was still in his police vehicle, and that later, after a brief chase, he fired the fatal shots at Mr. Brown because the teenager was coming toward him in a threatening way.

But while the precise timeline and exact circumstances of the shooting may never be fully known, several law enforcement experts challenged Officer Wilson’s assessment that nothing could have been done to change the deadly course of his confrontation with Mr. Brown.

From the time Officer Wilson first encountered Mr. Brown walking with a friend in the middle of the street on a hot afternoon in August, to the point the teenager lay dead on the pavement, there were several opportunities to de-escalate the confrontation, said Eugene O’Donnell, a professor at John Jay College of Criminal Justice and a former New York City police officer. Mr. O’Donnell pointed in particular to the initial moments of the confrontation, when the officer and Mr. Brown are said to have struggled through the open window of the officer’s police cruiser.

“There certainly wouldn’t be a prohibition of him driving a little further along and regrouping, calling for help and thinking about nonlethal weaponry,” Mr. O’Donnell said, referring to Officer Wilson. “Just because you’re a police officer doesn’t mean you have to go into a situation headfirst.”

Officer Wilson, whose detailed, four-hour grand jury testimony was among the evidence made public this week, contends that he was caught up in a rapidly escalating confrontation that started as a routine police stop and quickly spun out of control. Mr. Brown, he said, essentially pinned him in his police cruiser, holding the door shut while punching him in the face. He said he considered an array of responses, including using pepper spray or his baton, but found them all lacking.