As Germans struggle to make sense of the terrorist plot foiled this week, they are learning that their good Muslims - the large Turkish immigrant population here - may not all be so good.

One of the three militants arrested Tuesday and accused of planning major bomb attacks against American and German targets is a Turkish man who lives in Germany, and security officials said Friday that three, and possibly more, of the suspects still being sought are of Turkish origin.

The Turkish dimension of the plot has shaken Germans, who have long taken comfort in the belief that their Muslim population, predominantly Turkish, was less prone to terrorism, or even radical Islamic ideas, than Muslim minorities in Britain, France and other European countries.

This is not the first time Turks have been suspected of terrorist activity here. Last summer, the police in the Ruhr Valley detained eight people, most of them German Turks, on suspicion that they planned to bomb a concert by the pop singer Nena. They were later released without charge.

But this case - involving a terror network that allegedly stretched from southern Germany to Istanbul and Pakistan - bears a disquieting similarity to the bombing attack by British-born Pakistanis in Britain in July 2005.

"German officials don't like to talk about this, because it is politically sensitive, but they are genuinely afraid," said Guido Steinberg, who advised the previous German chancellor, Gerhard Schröder, on terrorism.

"We profited from the fact that most of the terrorists are Moroccans, Algerians or Kurds," said Steinberg, a researcher at the German Institute for International and Security Affairs in Berlin. "But we've got about two million Turks who are a theoretical recruiting base, and that's a critical mass."

There are, in fact, 2.7 million people of Turkish descent in Germany, 900,000 of whom are German citizens. Most are well integrated into German society, and very few fit the picture of militant would-be warriors, festering in angry isolation or yearning for martyrdom.

Yet experts say extremism is taking root - not just in Turkish enclaves but in proper German towns like Langen, outside Frankfurt, where Adem Yilmaz, the 28-year-old Turkish suspect, spent his teenage years.

The Yilmaz family lives in one of a stout troop of stucco apartment houses near the city library. On the gray, unadorned block sit a mix of German and Turkish names on yellowed mailbox labels. A woman who answered the door was not wearing a head scarf; she declined a request for an interview.

"At first he looked like a normal boy," Erich Rang, who lived in the apartment below the family, said of Yilmaz. "Then all of a sudden you could see him with a big beard, like the Taliban."

"There was a time when you didn't see him and people said he was doing his military service in Turkey," Rang said.

In fact, German intelligence sources say that Yilmaz was at a training camp in Pakistan in March 2006, run by the Islamic Jihad Union, a splinter group of a terrorist organization from Uzbekistan.

Turkish leaders expressed shock and dismay at the apparent involvement of so many Turks. They condemned the plotters and heaped praise on the German authorities for their police work. But some said they now feared a backlash against the broader Turkish population.

"Now, when Germans see a young Turkish man going to evening prayer, will they say, 'Aha, this is a sign of the radical drift of the Turkish community in Germany?' " said Sedef Ozakin, a Turkish-German woman who is a member of the city council of Munich.

She acknowledged that there were signs of radicalism among a small number of mostly young Turks. It manifests itself in two ways, she said: religious fervor, on the one hand, but also less well-known, a heightened sense of Turkish nationalism, evident in the T-shirts and hats emblazoned with the Turkish flag that are popular among Turkish-German teenagers.

The solution to this, Ozakin said, is for mainstream Turkish organizations in Germany to reach out to the young - to prevent them from being led astray by radical or violent ideologies.

The trouble is that established Turkish groups - which have close links to the Turkish state and were founded in the early years of immigration - are not attractive to third- and fourth-generation immigrants.

Many of these people were born in Germany or, like Yilmaz, spent much of their lives here. Such people, experts say, have little interest in the cloistered world of tea-drinking, suit-wearing men that one often finds in the inner courtyards at mainstream Turkish mosques.

Instead, they are attracted to smaller independent mosques, which are often more radical, according to Dirk Halm, a researcher at the Center for Studies on Turkey at the University of Duisburg-Essen.

Precise numbers are hard to find, but experts say the number of these mosques is rising. More than half the mosques in Berlin do not belong to traditional Turkish mosque organizations, Halm said.

This generation gap appears to be the case with Adem Yilmaz. The imam of the local mosque in Langen, sponsored by the Turkish government, said the father attended services frequently there but that the son did not. The mosque - a white house with a traditional tiled roof, and Turkish and German flags hanging outside - fits in nicely with the local architecture.

The imam, Hasan Ozdemir, said he was aware there were radicals in the region. He said he forbade one young man from coming to the mosque, he said, "after we determined that he was not representative of our religion and did not follow the philosophy of Islam."

"We are completely shocked," Ozdemir said of Yilmaz's alleged involvement. "We are a part of the society here in Germany. We live here. We pay taxes. It affects us deeply."

The Turkish links to terrorism come at a time of increasing tension between even mainstream Turkish leaders and the German government. In July, four major Turkish lobbying groups boycotted a meeting called by Chancellor Angela Merkel to foster better integration policies.

The leaders said they were protesting new immigration laws that give precedence in access to job-placement services to citizens of Germany and other European Union states over non-German Turks.

"If you say to people that 'you, as Turks, will not be treated equally,' it fosters disappointment and feelings of hopelessness," said Bekir Alboga, director of inter-religious dialogue at the Turkish-Islamic Union for Religious Affairs, one of the largest Turkish groups in Germany.

Alboga acknowledged that young Turks are less receptive to the message of mainstream Turkish groups. But he laid a share of the blame with the German government, which he said has not given the groups adequate funds to organize effective youth programs.

Adding to the tension, the efforts of Turkish groups like Alboga's to build prominent mosques in Cologne, Munich and other cities has run into resistance from local citizens' groups.

None of this friction, by itself, can account for the radicalization of young Turks. But experts say that Germany's failure to better integrate its Turkish minority has swollen the ranks of those Turks living in Germany who have no interest in participating in mainstream society.

"The guys behind these acts don't care about integration," Steinberg said. "They care about the killing of Muslims in Iraq or Palestine. They look at what our troops are alleged to be doing in Afghanistan."