A day after, I was struck by the response of Constable Lam. Bystander video put the viewer in his shoes as he de-escalated the situation and arrested the suspect without firing his gun. I worked with Catherine and the video team in our New York headquarters to break down his response moment by moment and show the concrete steps he took. It was a lesson in how police are trained to turn down a heated situation.

Rick Gladstone: I knew coming into work on Tuesday that my assignment would be stitching together material provided by our reporters in Toronto. I was ready for a day of chilling details from the police, witnesses and experts about a terrorist conspiracy, in which we explained how a radicalized suspect named Alek Minassian had replicated the mayhem we had seen in vehicular attacks by Islamic State disciples in Europe and New York.

Instead, we learned that the suspect appeared to be a sexually frustrated, woman-hating loner who had paid homage to a misogyny netherworld in a Facebook post, either before or during the attack. I never had anticipated Canada would be the country that led me to become familiar with misogynistic code words like incels, Chads and Stacys.

So what began in the morning as a news story about Mr. Minassian’s appearance in court, where he was charged with 10 murders, soon morphed into a profile of him. We also used the expertise of our social media reporters in New York, who helped us connect the dots between Mr. Minassian’s last Facebook posting and a 2014 killing rampage by a 22-year-old man in California that had become a perverse beacon of inspiration for misogynists.

Writing the story became an exercise in filling in the blanks without going beyond what we knew. It took a barrage of email exchanges, phone calls and text messages to nail down Facebook’s confirmation that the misogynistic posting was Mr. Minassian’s. It took at least three phone calls and reporting in Toronto to specify what we could say about the genders of the victims.

It was only after the adrenaline and tension began to ease toward the end of the day that I realized we had assembled a real-life horror story.