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-Qaida figures who allegedly 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 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 show Lashkar’s appeal to Americans and intensify fears that the group shares al-Qaida’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-Qaida 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 the Prophet Muhammad.

Despite stepped-up security, Headley was able to talk his way into the newspaper’s offices, according to an FBI complaint.

“This is what Danish intelligence was most scared of,” said Morten Skjoldager, author of “The Threat Within,” 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’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 alleged actions reverse that trend.

“It’s a bit surprising,” said Louis Caprioli, an executive at the GEOS security firm in Paris and former French anti-terror 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.”