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 enlisted them for terror 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, India. 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 underscores 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 Muhammad. Police in 2007 arrested two South Asians for manufacturing bombs in a Copenhagen apartment and last February broke up an alleged homegrown plot in which three suspects planned to assassinate the newspaper's cartoonist. Later, an al-Qaida car-bombing at the Danish embassy in Islamabad, Pakistan, 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 visiting the newspaper's offices in January on the pretext of wanting to advertise his immigration consulting company in Chicago, 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, a Long Island high school dropout who pleaded guilty this year to conspiring with al-Qaida 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 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. Radicalization among U.S. Muslims remains rare compared even with 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 others, or even to some of their 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 outfit 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.' (Begin optional trim) Headley was born Daood Gilani in the United States and attended military school in Pakistan, his family homeland. He admits changing his name in 2006 to 'raise less suspicion' when traveling, the complaint says. He also told FBI agents that he underwent training with Lashkar and had worked with the group for at least three years, authorities say. Created by Pakistani security forces as an arm in the struggle for Indian-occupied Kashmir, Lashkar funnels recruits to al-Qaida and participates in plots against the West. Lashkar's English propaganda appeals to aspiring holy warriors in North America and Britain. Foreigners find it easier to reach Lashkar training camps because they are tolerated or supported by elements of Pakistan's security forces, according to Western anti-terror officials. (End optional trim) In another case, two U.S. men convicted on terrorism charges in Atlanta this year were part of a network of Britons, Canadians and Americans who were radicalized by Lashkar and traveled to its camps. Headley developed the Denmark plot with a Lashkar operative in Pakistan and with Ilyas Kashmiri, a notorious militant chief who runs a training camp in Waziristan and has become a close al-Qaida ally, the complaint says, citing surveillance and Headley's confession. FBI agents arrested Headley Oct. 3 at O'Hare International Airport in Chicago as he began a trip to Pakistan to meet with Kashmiri, the complaint says. Militant groups remain eager for U.S. recruits because of their operational value, investigators say. Headley's handlers made the most of him as an undercover operative, at one point communicating with him about switching from Denmark to a plot in India, the complaint says. During the exchanges, Headley allegedly used code, substituting business terms for terrorism and company names for terror groups. 'The main thing is the business must go on,' he wrote on Sept. 20, according to the complaint. 'I don't care (if) I am working for Microsoft or I am working (for) GE or Philips.' The true meaning, according to the complaint: Headley did not care which group he worked for as long as he could help carry out attacks. 