PARIS — When 12 people were killed at the satirical newspaper Charlie Hebdo in Paris in January 2015, Mickaël Harpon — a future killer himself — declared, “serves them right.” A shocked colleague in the city’s Police Department heard him, and a sharp argument ensued.

But that was nearly as far as the complaint went. Colleagues of Mr. Harpon, who slashed four co-workers to death last Thursday, preferred “restraint,” a leaked internal police report said, and their bosses never pressed them. That summer, the police appeared to have had a chance to stave off the computer technician’s attack. They missed it.

Overlooked warning signs are now at the center of fierce recrimination, finger-pointing and calls for increased vigilance in a country that had half-forgotten the terrorist threat from within and that has suddenly reawakened to it.

In a somber, rain-soaked ceremony, President Emmanuel Macron honored the four dead on Tuesday in the courtyard of the massive Police Préfecture building, calling on the French to create a “society of vigilance.” It was in that sprawling space that a young police intern six days on the job pulled out his gun last Thursday and ended Mr. Harpon’s seven-minute rampage.