Security experts warned that the authorities needed to strike a balance between increased vigilance and avoiding religious discrimination in a country where suspicions against Muslims have festered, especially since a series of terror attacks in 2015.
“If they systematically flag the smallest events, there will be some abuses,” said Jean-Charles Brisard, president of the Center for the Analysis of Terrorism, a research group based in Paris. Mr. Brisard said the police over all had been alert to radicalization, adding that the headquarters attack had most likely been an isolated case.
And flagging too many things can be counter-productive, said Marwan Muhammad, a statistician and former spokesman for the Collective against Islamophobia in France. “When there are too many so-called weak signals, too much noise, there’s no intelligence anymore,” Mr. Muhammad said.
Mr. Lallement acknowledged to lawmakers that he had never expected a threat to come from within. “During the attack, I myself thought that we were being attacked from the outside,” he said.
The trauma from that day is still palpable when officers walk by the staircases and other spots where their colleagues died, he said.
Eric Poulliat, a lawmaker who was an author of a report this year on radicalization in the public service and who questioned Mr. Lallement on Wednesday, said the surge of reports showed that officers had realized they could be the target of more such “blue on blue” attacks.
Mr. Poulliat’s report found that of 150,000 police officers nationwide, 28 were being monitored for suspicious activities as of June.