On May 25, a 21-year-old soldier named Cédric Cordiez was stabbed in the neck in the La Defense district of Paris. He survived, but the aggressor’s intention was clearly to kill him (possibly even to sever his head). Four days later, a suspect referred to as Alexandre D., a 22 year-old-convert to Islam, was arrested. He confessed to having acted “on religious grounds.”

The Cordiez case is quite similar to the public killing and beheading in London of Lee Rigby, a British soldier, on May 22. One of Rigby’s murderers, Michael Adebolajo, a 28-year-old British-African convert to Islam, claimed to have acted in order to retaliate against the British military operations “against Muslims” in Afghanistan. “It is an eye for an eye, a tooth for a tooth,”  he said before being arrested by the police. “We swear by Allah Almighty that we will never stop fighting you.”

Manuel Valls, the French interior minister, drew another, even scarier, parallel: he mentioned Mohamed Merah, the deadliest jihadist terrorist to have operated in France so far.

A French citizen of Algerian descent, Merah shot eight people in eight days last year, from March 11 to March 19: seven were killed on the spot, one survived as a quadriplegic. The victims were selected according to clear criteria. Merah first targeted “defectors “: young men of North African or Caribbean origin serving in the French military (and thus likely to fight or to have fought other Muslims in places like Afghanistan or Mali). Then he murdered Jews (since Jews are deemed to be, as a race, enemies of Islam): three preteen children and a teacher at a Jewish school.

“There are several dozens, perhaps even several hundred, potential Merahs in our country,” Valls somberly observed during a press conference on May 29. Indeed, investigations linked to the bombing on September 19, 2012, of a kosher shop in Sarcelles led the police, one month later, to a ramified Islamist network involved in stockpiling weapons and explosive material and in gathering information about Jewish personalities and organizations. Sources say that several other networks have been found since then.

However, some wonder whether the French police and security agencies are indeed “discovering” radical and seditious groups and individuals or just dealing more seriously with groups and individuals they already knew. The fact is that both Merah and Alexandre D. had been followed closely for years and identified as security risks prior to their crimes. The police and the agencies were aware that Merah had extensively traveled to no less than 25 Middle Eastern, Central Asian, Far Eastern, and African countries, including Afghanistan. They even had been briefed negatively about him by the Pakistani and American security agencies. Nevertheless, they had not taken steps against him.

As for Alexandre D. — or Abdelillah, as he insisted on being called after his conversion — Le Monde reports that he was put on file as early as February 20 by SDIG, the special branch of DCRI (the domestic security agency) that monitors Islamist activities. Nothing was done to prevent him from taking action.

Why not? One answer is that democratic countries are not supposed to arrest or intern citizens on the mere suspicion that they might be involved in crimes in the future. Another answer is that preventive action can be counterproductive, as any police department knows: as soon as you arrest suspects, other suspects, or criminals yet undetected, go into hiding.