WASHINGTON -- The arrest of three terror suspects in Germany yesterday followed months of monitoring by American authorities concerned about an intensifying threat to the United States, according to senior U.S. counterterrorism officials.
German officials informed the U.S. government at the end of last year that American military facilities were being targeted, an intelligence official said. U.S. interest in the alleged plot became "much greater at that point."
While many details have yet to emerge, terrorism analysts said the German plot could represent a new and dangerous turn in the radicalization of local youths in Western countries.
Top U.S. security officials said the most recent arrests, as well as a foiled plot in Denmark announced Tuesday, reinforced their concern about the heightened danger of a terrorist strike that intelligence reports have warned about in recent months.
"Arrests in Denmark and Germany indicate that al-Qaida continues to carry out active war against the West," Homeland Security Secretary Michael Chertoff told a House panel. "They continue to seek fellow travelers and allies and adherents in the West who can be used to carry out attacks, whether they be in Western Europe or here in the homeland."
U.S. officials said the German threat, which included an alleged plot to attack a U.S. military base in Germany, stood out as particularly serious.
One senior official called it a "plausible and imminent" threat, and another described it as "very legitimate." Several U.S. officials spoke on condition of anonymity to avoid upstaging German counterparts.
Intelligence officials and terrorism specialists said the arrests of the suspects -- two German converts to Islam and a Turkish man, all in their 20s -- reinforced growing concerns about radicalization taking hold in Western countries.
German prosecutors alleged yesterday that the three men had obtained 1,500 pounds of hydrogen peroxide that could have been used to make explosives. Officials said the suspects were part of a German terrorist cell that had been trained in Pakistan at camps organized by the Islamic Jihad Union, a radical group based in Uzbekistan.
The Uzbek group, known for its close ties to al-Qaida, "distinguishes itself through a profound hatred of U.S. citizens," Joerg Ziercke, who heads Germany's Federal Crime Office, told reporters in Berlin, according to news reports.
German Interior Minister Wolfgang Schaeuble said the three men were inspired at least in part by an Internet recruitment effort but also got "highly professional training in Pakistan," the German news agency DDP reported.
If it proves true, the cycle of Internet recruitment, training in Pakistan and returning home to launch an attack would be a first, according to Roger Cressey, a former senior U.S. counterterrorism official. "That would be a very big deal and a very bad development," he said.
Also notable, terrorism experts said, was the apparent link to the Islamic Jihad Union.
The Uzbek group "has become, in a sense, a service provider to the jihadist enterprise," said Brian Jenkins, a terrorism specialist at the Rand Corp., who has written extensively on terrorist recruitment.
The ability of German citizens to link up with training camps in Pakistan, Jenkins said, suggests that there is an underground network linking the two countries. The key ingredients that transform sympathizers into active terrorists, he said, are usually training and the acquisition of weapons to carry out an attack.
Cressey said he could not recall another occasion in which a European terror cell professed allegiance to the Islamic Jihad Union, which historically has focused on trying to establish an Islamic government in Uzbekistan.
The Uzbeks and the Russian separatist Chechens "are believed to be part of the [al-Qaida] inner circle these days," Cressey said.
The Homeland Security Department has been following the investigation closely, said spokesman Russ Knocke, and "at this time, there's no information telling us of an imminent threat to the homeland."
The U.S. government "is not adjusting its security posture," Knocke said, though it notified state and local officials of the German arrests. Airports in the United States remain at an "orange" or "elevated" threat level, which was instituted in August 2006 after an alleged British terrorist plot to shoot down U.S.-bound jetliners was foiled.
In April, the U.S. Embassy in Berlin warned that "U.S. diplomatic and consular facilities in Germany are increasing their security posture" in response to "a heightened threat situation."
Though German officials would not confirm the information in a briefing with reporters yesterday, Frankfurt's international airport and the U.S. Ramstein Air Base were the targets of the plot, according to a German television report that cited unnamed officials as sources.
The German arrests followed an announcement Tuesday by Danish officials of an alleged plot in which eight men, who included Pakistanis and Afghans, were arrested. Authorities have not described the nature of that plot.
Terrorism specialists noted parallels between the emerging details of the German and Danish arrests and recent attacks, including the crudely executed airport bombing in Glasgow, Scotland, in June, the London subway bombings in 2005 and the train bombings in Madrid, Spain, in 2004. In each case, a combination of homegrown radicals and outside training converged in an attack in the terrorists' home countries.
"An unknown question is, what radicalized them?" Cressey said. "You see a trend in Europe that has the potential to spill over here if the self-radicalization phenomenon and recruitment over the Internet continues in this direction."
As radical groups emerge in Europe, another major concern is the ease with which their members could come to the United States.
The Homeland Security Department is working to close a security gap in a program that allows residents from 27 countries, including many in Europe, to come to the United States without a visa, Knocke said.
The government is developing a mechanism that would require residents of countries that are part of the waiver program to submit background information to the Homeland Security Department through a Web site, so background checks can be conducted on those who plan to fly to the United States. He said the new system could be in use within months.