PARIS — Last June, Patrick Calvar, the head of France’s domestic intelligence service, faced a decision: continue surveillance on a French Islamist who had been viewed as a potential threat for a decade, or shift limited resources to help monitor a swelling new generation of fighters returning from Syria.
The surveillance on the Islamist, Saïd Kouachi, had turned up nothing for over two years, and monitoring of his younger brother, Chérif Kouachi, had been abandoned the previous year, French officials say. Earlier in 2014, the intelligence service had transferred Saïd Kouachi’s case for several months to the Paris police, a sign that it was no longer considered a priority.
The three-member commission scrutinizing requests for cellphone monitoring by the intelligence agency had signaled that its recommendation would be against further surveillance. And the prime minister, Manuel Valls, was under intense pressure to focus on what seemed to be the more immediate threat emanating from Syria; the previous month, Mehdi Nemmouche, a 29-year-old Frenchman who had fought in Syria, had gunned down four people at a Jewish museum in Brussels.
The counterterrorism team reporting to Mr. Calvar, a longtime intelligence official, allowed the surveillance order on Saïd Kouachi to expire. Less than seven months later, the Kouachi brothers burst through the doors at the French satirical newspaper Charlie Hebdo and fatally shot 12 people, setting off a two-day manhunt that involved a third gunman and ended with another five victims and the deaths of all three gunmen.
The decision to drop surveillance of the Kouachis was one in a series of developments that, in the aftermath of the deadliest acts of terrorism in France since Algeria’s struggle for independence in the 1960s, suggests substantial failures or weaknesses in French intelligence and law enforcement.
It also highlights security challenges facing other Western governments, as Denmark was reminded this weekend when a native-born Muslim gunman in Copenhagen killed two people in an attack that had numerous similarities to the rampage in and around Paris last month.
Largely caught off guard by the proliferation of potential threats, they now confront wrenching trade-offs in deciding how and whether to monitor hundreds or thousands of their citizens who are traveling in and out of conflict zones, otherwise making contact with radicals or being inspired by assaults like the one on Charlie Hebdo.
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But both Kouachis gradually slipped down the priority list as the authorities scrambled to deal with the largely unforeseen effects of the civil war in Syria and the rise of the Islamic State. The number of French fighters returning from Syria was climbing rapidly: Some 1,400 people in France are believed to have either joined the jihadist cause in Syria and Iraq or sought to do so.
By law, the domestic surveillance powers of French intelligence agencies are limited. Wiretaps are still governed by rules drafted in 1991, long before cellphones and the Internet became ubiquitous. French intelligence agencies cannot legally track cars or bug apartments in their own country. Since 2006, they have had some access to the metadata of electronic communications, but they cannot spy on the content of emails.
But all along, there had been an alternative means of tracking the Kouachis. The judicial system has long been active in counterterrorism and has considerable flexibility to open investigations into anyone suspected of potentially carrying out a terrorist act.
Mr. Trévidic, the chief terrorism investigator in the judicial system, said if the domestic intelligence agency had wanted to turn the case of the Kouachis over to him, it probably could have. Since the Merah case in 2012, twice as many terrorism cases have been transferred from the intelligence services to become judicial investigations, he said.
“The tools are there,” said François Heisbourg, a former defense official and counterterrorism expert. “But the authorities did not bring all their tools to bear on people who had exactly the profile they said they were worried about.”
Katrin Bennhold reported from Paris, and Eric Schmitt from Washington. Aurelien Breeden and Laure Fourquet contributed reporting from Paris.
A version of this article appears in print on February 18, 2015, on page A1 of the New York edition with the headline: Gaps in Surveillance Are Clear; Solutions Aren’t. Order Reprints| Today’s Paper|Subscribe
