WASHINGTON — It is a worrisome first: an American accused of going to Europe to plot a terrorist attack there.
Recent arrests in Chicago underscore a growing concern among Western officials about the threat posed by U.S. militants who take advantage of their passports to travel easily around the world on violent missions.
"We never thought it could be persons from the U.S. coming here to commit attacks," said Hans Jorgen Bonnichsen, a former chief of Denmark's police security intelligence service. "This shows a new tendency."
The Chicago case centers on David Coleman Headley, a Pakistani American businessman who allegedly traveled to Denmark to plot an attack on a newspaper targeted by Islamic extremists because it published cartoons of the prophet Muhammad.
Headley, 49, becomes the latest of several U.S. citizens recently accused of direct contact with top Al Qaeda figures who enlisted them for terrorist plots. But he also stands out because he is older and more sophisticated than suspects in previous cases and, according to investigators, used his consulting business as a cover for clandestine militant activity overseas.
Headley also allegedly conspired with Lashkar-e-Taiba, the Pakistani militant group accused of carrying out last year's bloody, highly organized attacks in Mumbai. Those contacts intensify fears that the group shares Al Qaeda's determination to strike the West.
With officials saying additional arrests are possible, the case also reiterates a surprising reality: One of the world's most likely targets of terrorism today is placid Denmark, population 5.5 million.
"Until yesterday, the threat was mainly from homegrown groups," Bonnichsen said in a telephone interview. "This case shows a very strong connection to Al Qaeda groups in Pakistan. That is really a challenge and we can only handle it by depending on good international cooperation."
Denmark has confronted a barrage of propaganda and threats since 2005, when the Jyllands-Posten newspaper published caricatures of Muhammad. Police in 2007 arrested two South Asians for manufacturing bombs in a Copenhagen apartment, and in February 2008 broke up an alleged homegrown plot in which three suspects planned to assassinate the newspaper's cartoonist. Later, an Al Qaeda car bombing at the Danish Embassy in Islamabad killed six people.
Danish security forces keep close watch on their surprisingly fierce extremist underworld. But they had not expected the likes of Headley, who admits having visited the newspaper's offices in January on the pretext of wanting to advertise his Chicago immigration consulting company, according to an FBI complaint.
In January and during a second trip in July, Headley filmed video of potential targets during alleged scouting missions in Copenhagen and Aarhus for what officials say may have been a commando-style raid like the Mumbai attack.
Despite stepped-up security, Headley was able to talk his way into the newspaper's offices, according to the complaint.
"This is what Danish intelligence was most scared of," said Morten Skjoldager, author of "The Threat Within," which is about terrorism in Denmark. "The extremist environment in Denmark is so small that if you get in touch with someone in that world, it will be noticed by the intelligence services. But so far it seems he had no connections with Denmark."
Headley seems an especially effective operative because he does not fit the profile of the typical Islamic militant. He is older than suspects in other cases, such as Najibullah Zazi, the 24-year-old Afghan American charged last month with preparing bomb attacks in New York.
And in contrast to Bryant Neal Vinas, the Long Island high school dropout who pleaded guilty this year to conspiring with Al Qaeda leaders in Pakistan, Headley's purported work as an immigration consultant gave him an air of respectability. Nonetheless, the FBI complaint alleges that his company did little business and may have been just a front.
U.S. authorities have long feared that terrorists forged in Europe's large militant communities could try to take advantage of limited visa requirements to enter the United States and carry out an attack. Headley's travels reverse that concern.
Radicalization among Muslims remains rare in the U.S. compared even to small countries like Denmark. Western officials assume that Americans would be used by Islamic militants for attacks here. In Europe, police generally devote less scrutiny to U.S. visitors than to others, even to some of their own citizens of immigrant descent returning from South Asia or North Africa.
"It's a bit surprising," said Louis Caprioli, an executive at the GEOS security firm in Paris and former French anti-terrorism chief. "It's the first time we talk about an American leaving for Europe for a terrorist act. Maybe the United States is becoming a factory for terrorists."