Armed with few means but plenty of ideological fervor, an emerging corps of wannabe terrorists is scoping out skyscrapers, conducting terror-training camps, and, in one case, even attacking Americans by using a Jeep.

The June 22 arrests of six men in Miami and one in Atlanta for plotting to destroy Chicago's Sears Tower and public buildings elsewhere provide the latest case in point. The band of alleged terrorists - which the government says pledged fealty to Al Qaeda but had no actual contact with it - has been characterized as "homegrown" because five of the suspects are US citizens.

From the Ku Klux Klan to the Weather Underground, American society has reaped its share of violent dissident groups. The government alleges that this latest group embraced jihad against the US, though it appears to have blended radical Islam with other religious beliefs. Even so, the arrests since 9/11 of some 50 Muslim-Americans - most of them born and raised in the US - is of growing concern to law enforcement and terrorism experts.

Most are amateurs, but law officers say several factors set this new threat apart: their means of recruitment, the source of their anger, their lack of direct tie to international terror groups, and their connection via the Internet to an apparent transnational web of sympathizers.

"These homegrown terrorists may prove to be as dangerous as groups like Al Qaeda, if not more so," FBI Director Robert Mueller told the City Club of Cleveland in a speech Friday. "[They] are self-recruited, self-trained, and self-executing. They may not have any connection to Al Qaeda or to other terrorist groups. They share ideas and information in the shadows of the Internet. They gain inspiration from radical websites that call for violence."

On Thursday, the group in Miami, mostly of Haitian descent, joined the ranks of those linked to foiled terror plots since 9/11. Though lacking resources, the men had told an undercover agent that they were planning an attack more spectacular than those of 9/11, according to the government indictment.

Other recent arrests include:
• Two Georgia men arrested in March, both Muslims, are accused of being involved in international terrorism activities, making greater Atlanta the site of three recent antiterror arrests. Authorities have linked them to those arrested in Canada in an alleged plot to attack targets in Toronto.

• In Torrance, Calif., a group of gas- station burglars were arrested last summer and charged with plotting to finance the destruction of military-recruitment offices. Three of the four are US citizens.

• Though not classified by the FBI as a terrorist act, a Muslim student's March rampage across the University of North Carolina quad in his Jeep Cherokee injured nine people. He is reported to have told investigators he was "avenging the death of Muslims all over the world."

Experts cite cultural alienation, the influence of radical clerics, and even youthful rebellion run amok as motivations for these plots and misadventures. But what ties many of them together is the idea of defending a religion under attack.

"One fundamental common denominator of these cases is the belief that there's a war against Islam and therefore you have to avenge it," says Steven Emerson, author of "American Jihad: The Terrorists Living Among Us."

Many of those arrested were not seen as threats by those who knew them. In an earlier case in Lackawanna, N.Y., the six men who ultimately pleaded guilty to providing material support to terrorists grew up watching football, played sports, and enjoyed barbecue. In the Miami case, one of the suspects is a musician who also worked at an Abercrombie & Fitch store.

"These folks have various jobs, including ice-cream truck driver. A lot of them are students, a lot of them are Muslim converts or their parents came from Middle Eastern countries," says Oren Segal, director of the Center for the Study of Left-wing and Islamic Extremism in New York.

Particularly disturbing for law enforcement is that so many, like the London subway bombers last summer, are citizens without rap sheets whose violence seems to come out of nowhere. The FBI's Mr. Mueller said Friday that the bureau met with Muslim organizations as recently as last week to discuss issues in a community with as many as 7 million members in the United States, nearly half of whom are African-Americans.

"You're getting people who aren't connected to particular groups, and they seem to be moving very rapidly from a period of having no criminal record to suddenly being willing to use violent methods to make a political point," says Gary LaFree, director of the National Consortium for the Study of Terrorism and Responses to Terrorism in College Park, Md.

Questions arise, however, about how much the government makes of some of these alleged plots, many of which seem amateurish. The Miami group, officials acknowledge, posed no immediate threat.

"You have a Republican Party that's perhaps in trouble, which needs to say, 'Look, we foiled this plan, we're keeping America safe,' " says M. Ali Khan, director of the American Muslim Council in Chicago. "But ... did they have the means to do what they were talking about?"

More worrisome is that, with so many amateur networks at play, a group of low-profile professionals can be lost in the shuffle. A lowball figure of Al Qaeda sympathizers living in the US is about 1,000, including perhaps 300 extremists, a former Homeland Security inspector general, Clark Kent Ervin, has said.

"We can laugh at these amateurish efforts, but it does seem that what's developing are elements of alienated Islamic populations in our own country now. We need to recognize that [these plots] are an early warning that we need to understand what the dynamic is," says Peter Sederberg, a political scientist at the University of South Carolina in Columbia.

In the Miami case, the peculiar set of influences at work underscores the challenges of infiltrating the fragmented terror network, law-enforcement experts say. The religious background of the men is an apparent mix of Islamic, Christian, Jewish, and martial-arts tenets. The group was inspired by a "violent jihadist message," US Attorney General Alberto Gonzales said last week.

"What we're seeing now is quite similar to the militia movement, where we had groups all over - in far greater numbers than with Islamists - plotting things," says Mark Pitcavage of the Anti-Defamation League. "But we managed to survive that pretty darn well because law enforcement was vigilant - and we have this same state of vigilance post-9/11."