The fatal encounter, which lasted less than two minutes, occurred at about noon on Aug. 9.

According to the report, Mr. Brown had just left the convenience store and was walking with a friend in the middle of a street when Officer Wilson, thinking they might be the suspects in the robbery, blocked their way with his patrol vehicle. After a scuffle in which Mr. Brown reached inside the car, punched Mr. Wilson and grabbed for his gun, the officer fired two shots, one of which grazed Mr. Brown’s right hand, causing the teenager to run away.

The officer pursued, but then Mr. Brown suddenly turned and headed back toward him. The officer fired 10 more shots in three volleys, according to witnesses and bloodstain patterns on the ground. After being hit with six to eight bullets, Mr. Brown was finally felled by a shot into the top of his head.

In a written statement, Mr. Brown’s parents, Lesley McSpadden and Michael Brown Sr., said they were saddened and disappointed with the news that “the killer of our son wouldn’t be held accountable for his actions.”