PARIS — In a stunning wave of arrests, the security forces of France, Belgium, and Germany are rounding up suspected jihadis all over the map, especially those who have returned from the Syrian and Iraqi war zones.

In one case, in the small Belgian town of Verviers near the German border, two alleged jihadis were shot dead and one was wounded in a Thursday night firefight.

A spokesperson for the Belgian prosecutor’s office, Eric van der Sypt, said Friday that the Verviers suspects were believed to be on the verge of launching an attack. Four Kalashnikov automatic rifles were found in their possession along with bomb-making materials. Tellingly, they also had police uniforms. Phone taps of conversations among the suspects reportedly indicated the assault was only hours away.

“They had the intention to kill police, targeting them in the streets and at their offices,” van der Sypt said in Brussels on Friday. “We had been following the cell for a while but decided to intervene because the threat seemed imminent.”

He said this was a strictly Belgian cell, but all of this is taking place in the aftermath of the terror attacks in Paris last week, when known jihadis who had been under surveillance in the past somehow slipped the attention of law enforcement, acquired weapons of war (reportedly in Belgium), and launched a killing spree that took the lives of 17 victims, including journalists at the satirical weekly Charlie Hebdo, police, and Jewish shoppers at a kosher grocery.

What is clear is that the authorities in Europe now believe it is too dangerous to let potential terrorists who have fought and trained abroad continue to roam the streets. French Prime Minister Manuel Valls, in the aftermath of last week’s attacks, said flatly that his nation is in a state of war.

The national sense of vulnerability was heigtened on Thursday when officials announced that as many as 19,000 French websites had fallen prey to relatively primitive hacking and pranks since January 10. "It's the first time a country is confronted with such a large wave of cyber protest," Vice-Admiral Arnaud Coustillière, head of national cyber defenses, told the press. "In this load of attacks, there are groups that are more or less structured whose names are known, and groups of Islamist hackers that are well known who have larger capabilities and who are in my opinion behind the denials of service," said Coustillière. Out of the 19,000 attacks, about 10 specifically targeted French defense ministry sites. Only two succeeded, targeting regiment websites of the army on January 9 and 12. "We are taking this crisis seriously," said Coustillière, insisting "we are absolutely not worried."

In fact, worry hardly begins to describe the concerns behind the arrests over the last two days. But the legal foundation for detaining suspects varies from country to country, and may create loopholes through which potential terrorist attacks similar to the ones in Paris can still be organized.

Alain Bauer, one of France’s leading criminologists and an expert on counterterrorism, tells The Daily Beast that there’s widening recognition that surveillance tactics and strategies will have to change.

“Counterterrorism used to be like counternarcotics,” says Bauer. “You wait and you wait, and then you get another guy, with the idea that you are working your way eventually to the boss. But time, which was the ally of counterterrorism in the past, is now the enemy.” In the old days, suspects were followed from training camp to training camp, from connection to connection, as authorities mapped out whole networks. But the Internet allows connections to be made very quickly, and inspiration for attacks to take effect without any direct connection at all.

In the wake of the Paris slaughter, the concern is partly that there’s a guiding hand somewhere abroad directing this violence in Europe. Al Qaeda in the Arabian Peninsula (AQAP) has claimed it sponsored the attack on Charlie Hebdo, specifically. The so-called Islamic State in Syria and Iraq (ISIS) has called for attacks on Western targets in retaliation for the U.S.-led coalition’s efforts to “degrade and destroy” its capabilities. Two of the Paris shooters said they were working for AQAP, but one pledged allegiance to ISIS.

There is also the danger posed by groups that answer to no authority, or follow only vague directives, which could have been the case of the killers in Paris despite the various claims. Other groups or cells, seeing the enormous impact of what happened last week, will simply try to copy them. And the sooner they act, the more impact they are likely to have, with international media and politicians ultra-sensitive at the moment. More than 40 world leaders and more than four million people marched in France last Sunday to show their united opposition to terrorism. U.S. Secretary of State John Kerry is in the French capital today to show Washington’s solidarity. All of which raises the stakes and, unfortunately, the profile of any new terrorist acts.

The threat in France is not considered to be over. Twelve people, nine men and three women, were detained here overnight in connection with last week’s attacks, mainly on suspicion they supplied some sort of logistical support. Under French law, the nebulous charge of association de mafaiteurs, association with wrongdoers, can be used for what amounts to preventive detention. Meanwhile, soldiers and police continue to patrol the streets of Paris, and even the waters of the Seine, as the country remains on the highest state of alert.

In Germany, some 250 police raided 11 apartments to break up a group thought to be linked to ISIS. Two men were placed under arrest, formally under suspicion for planning an attack in Syria and laundering money. German authorities did not link those detained to plots for attacks inside Germany or Europe.

Tobias Kaehne, a spokesman for the German criminal court in Berlin, told The Daily Beast’s Nadette De Visser that those arrested were not being held as a purely preventive measure. “It is difficult to put someone in prison on suspicion alone, if you don’t have proven facts,” he said. “We have the history of the Nazi regime, in which the citizens’ rights didn’t really exist, especially those of minorities.” Nobody wants that history to repeat itself.

But without strong measures aganst organized murderers, police find themselves scrambling to protect every possibe target. In Belgium, France, and Germany, for instance, Jewish schools are either being heavily guarded or temporarily closed down, lest they come under attack.

In Britain, crackdowns on suspected jihadis have been escalating for some time. In September there were a series of dawn raids targeting known terrorist sympathizers. The Daily Beast’s Nico Hines reports from London that one of those men subsequently skipped bail and turned up in the Islamic State with his baby and an automatic rifle, highlighting Britain’s inability to keep these people out of trouble.

Britain does not have preventive detention as such, but it does have something called Terrorism Prevention and Investigation Measures, which allows the Home Secretary to force suspects to wear electronic tags even though they have been found guilty of no crime. Nonetheless, some people have managed to escape.

Even the new radical proposals for heightened security measures in the U.K. don’t include preventive detention, although they would allow the government to relocate you to another city away from your suspected terrorist “cell.”

Hines reports “that doesn’t mean to say you can’t be charged and jailed for something vague like preparing for terrorism or inducing terrorism, e.g. by tweeting in favor of jihad.”

The maximum pre-charge detention, which was temporarily extended to 28 days after the 9/11 attacks, went back down to 14 in 2011.

In Italy, reports The Daily Beast’s Barbie Latza Nadeau, the government is watching 800 jihadi fighters who are “ready to attack,” according to today’s Espresso magazine, which got its hands on the secret security-service files detailing the allegations against these men and women. They will fall under the same laws as those that apply to the Mafia and organized crime, and, in fact, Espresso says the jihadi fighters use the mobs’ criminal networks to acquire arms and false documents.

Italy can hold anyone suspected of a crime up to a year without charging them, and can continue to keep them in jail if there is risk of flight or of their repeating the offense.

“Authorities here work on the theory that Italy is a corridor between the Middle East and Europe where guns and money can be smuggled with ease,” Nadeau reports, “and they are constantly trying to break those networks down.”

But the challenge is growing by the day.