CHICAGO — For days, officials here have faced scathing criticism for taking too long to share what they knew about the death of Laquan McDonald, who was shot 16 times by a police officer in October 2014 as a dashboard video camera captured it all. On Monday, the authorities seemed determined to make a sharply different impression as they released video of another fatal police shooting of a black man from the same month, which has also become a source of contention in the city.

In an hourlong presentation that included audio from 911 calls, detailed maps, witness interviews and dashboard video, the Cook County state’s attorney, Anita Alvarez, said that her office would not bring charges against Officer George Hernandez in the fatal 2014 shooting of Ronald Johnson. Ms. Alvarez said the array of material provided clear evidence that Mr. Johnson had been armed with a gun, resisted arrest and posed a potential threat to pursuing officers.

But the elaborate nature of the presentation also underscored the intense public pressure and distrust local officials, from the Police Department to City Hall, have faced since the release last month of the video of Mr. McDonald. That dashboard video outraged many Chicago residents by showing that Mr. McDonald was veering away from an officer when he was shot, and that the officer continued firing bullets into him after he had already fallen to the ground.

Image Ronald Johnson in June 2014. Mr. Johnson was 25 when he was fatally shot in October 2014 during an encounter with Chicago police officers. Credit... Dorothy Holmes, via Associated Press

Scrutiny of the Chicago Police Department is also coming from the United States Justice Department, which announced on Monday that it had opened a sweeping investigation into possible civil rights abuses, including whether there have been racial disparities in the department’s use of deadly force.