Dozens of investigators scoured the crime scene in northern France. More than 450 police officers combed the countryside and the surrounding area. Interpol issued an alert.

French officials said they would “spare no effort or means” to track down heavily armed assailants who ambushed a prison convoy in a brazen daytime attack, killing two guards and freeing an inmate.

But three weeks into an extensive manhunt, the suspects are still on the run.

The case has raised uncomfortable questions about whether France’s justice system fully grasped how dangerous the inmate was and if its overburdened prisons had played a role.

The authorities have been tight-lipped, declining even to specify how many people participated in the attack. But they say their investigation has made progress.

Laure Beccuau, the top Paris prosecutor, told Franceinfo radio last week that the authorities had “a number of leads that I would describe as serious.” She did not elaborate, saying only that the ambush had been well-organized, and that the suspects appeared to have planned hide-outs.

The attackers vanished in stolen cars that were later found burned. Experts say it is only a question of when, not if, they are captured.