Chicago attorneys pose as reckless officer while filming Laquan McDonald

At Monday's courthouse, the focus was on a few minutes of tension almost four years ago, and not on the systemic issues facing the department. The trial started swiftly, with each party presenting its opening file and eight witnesses, including an officer who had followed Laquan before Officer Van Dyke arrived and opened fire.

This officer, Joseph McElligott, said to have remained about 15 feet behind Laquan during a slow-moving pursuit. Officer McElligott took out his gun and repeatedly told Laquan to drop his knife, but he never fired a shot. His partner stayed in their car and radioed a colleague with a stun gun.

They continued to follow from afar, even after Laquan put a tire on their police car and hit the windshield.

"We were trying to save time for a taser," said Constable McElligott. "He did not make any direct movement on me. And I had the impression that my partner was protected for the most part inside the vehicle.

Another officer, Dora Fontaine, who arrived at the scene just before the shooting, said she saw Laquan walking down the street with a knife in her hand and that she had heard another officer tell her to drop the gun. But she said Laquan never went to an officer or raised her arm. Like all officers on the scene, with the exception of officer Van Dyke, Agent Fontaine never fired a shot.

For years, the case against agent Van Dyke and the video of the camera triggered a tumult in Chicago and debated when the police had to use force. Mr. Emanuel fired the police commissioner and the rules governing when officers can fire are reinforced. The prosecutor who waited more than a year to file a complaint has lost his candidacy for reelection. All patrol agents were equipped with Tasers cameras and cameras.

Agent Van Dyke is charged with first degree murder, aggravated assault and official misconduct and could spend the rest of his life in an Illinois prison where he was convicted. But police have full discretion to use deadly force, and prosecutors in other cities find it difficult to convince jurors to sentence them.