The key events took place in a matter of one second. In an analysis, The New York Times slowed down the police video, as well as another of the 21 videos released by the authorities.

As the officer, identified in police reports as Eric E. Stillman, 34, fires the single shot, Adam is raising his arms and appears to be empty-handed. In the moment before the shooting, the Times’s analysis shows, Adam can be seen holding what appears to be a gun behind his back, which he drops behind a wooden fence just before he raises his hands.

After firing the shot, Office Stillman called for an ambulance, searched for the wound and began CPR. “Stay with me,” he said to Adam more than once.

The Civilian Office of Police Accountability, an independent agency that investigates police shootings in Chicago, released the videos on Thursday after initial resistance to making them public, citing Adam’s age.

The Chicago Police Department had no comment on the video aside from redistributing its news release about the shooting from April 1, which called the loss of life “tragic” and said the department would cooperate with COPA, which is investigating the use of force.

A lawyer for Officer Stillman, who is white, said that the shooting, while tragic, was justified given the nature of the threat. “The police officer was put in this split-second situation where he has to make a decision,” said Timothy Grace, a lawyer at the firm of Grace & Thompson retained by the Fraternal Order of Police in Chicago.

Officer Stillman has been placed on administrative duties for 30 days; he joined the Chicago police in August 2015 after serving in the military overseas, his lawyer said.