He pointed an object threateningly at Constable Ken Lam, the Toronto police officer who was the first to encounter him as he stepped outside of his rental van.

Constable Lam pulled out his gun and commanded the man, 25-year-old Alek Minassian, identified by the police as the driver who had plowed down dozens of pedestrians on a busy Toronto street on Monday, to get down.

But before rushing toward him, or firing his gun, Constable Lam did something else: He paused, walked briskly back to his car and turned off its blaring siren.

Policing experts cited that as the first sign not only of Constable Lam’s calm-under-fire composure, but also, and more important, of the training he had in de-escalation tactics. There were many more.