PARIS - Political leaders and police investigators across Europe have concluded that the threat of terrorism from al-Qaida and other radical Islamic groups is more serious than they had earlier assumed and may take years to neutralize.
Recorded messages attributed to Osama bin Laden and his deputy, Ayman al-Zawahiri, that indicate Europe as a target have prompted politicians to put the continent on alert.
Recent arrests, particularly in France and the Netherlands, have underscored the extent of the problem. Testimony at trials of suspected al-Qaida members in the Netherlands and Germany has suggested the global reach of bin Laden's terrorist organization.
The holiday season has contributed to the edginess. In France, the prime minister announced Thursday that he was mobilizing "the entirety of state services" to combat potential attacks, and the Interior Ministry said it would double the size of its anti-terrorism unit to 800 soldiers and reinforce its police and undercover force.
Senior European officials dealing with terrorism say that recent investigations have uncovered surprisingly well-established networks of Muslim militants with potential to commit terrorist acts and affiliations that stretch across Europe to operatives in North America, North Africa, the Middle East and Central Asia.
But in the absence of hard intelligence pointing to a specific threat, the officials disagree sharply over the extent to which Europe is actually more vulnerable to terrorism since the Sept. 11 attacks. There are suspicions among counterterrorism experts that politicians and law enforcement investigators may be exaggerating the threat in order to appear vigilant in the eyes of the public and to cover themselves should an attack occur.
Pedro Rubira, the Spanish High Court prosecutor dealing with terrorist cases, is among those who say Europe is at greater risk. "Arrests might shut down a cell, but it's impossible to destroy it completely," he said in an interview.
European investigators have said that Islamic terrorists are often hardly different from small-time racketeers turned radical, who get money from the sale of contraband and form into small, mobile units. But some terrorism experts say that portrayals of the threat are being shaped by politics.
"Historically, we French are in the camp of skeptics when it comes to terrorism," said one senior government official. "On the one hand, we have the same intelligence we've had for some time that something big may happen in the heart of Europe. On the other hand, we have a turn to the right in this country, and we don't want to appear to be taking the threat less serious than President Bush or [British Prime Minister] Tony Blair."