The threat of "home-grown" terror cells is taking on new urgency after Canadian police said they broke up a major terrorist bomb plot just across the border in Toronto by arresting 17 men and youths mostly born in Canada.
The fear among analysts is that the Canadian cell represents a new threat "home-grown" Islamic radicals like those who bombed trains in Madrid and London.
"The structure of al Qaeda as we know it has now been destroyed," said Jack Cloonan, a former FBI counterterrorism official and an ABC News consultant. "It's no longer a top-to-bottom organization. … They don't need direct control from al Qaeda, so that makes it a lot more difficult for us to counter."
It's a threat the FBI Director Robert Mueller warned about back in April.
"Today's threat is just as likely to come from our own streets," Mueller said, "as it is to come from persons who are sent here from overseas."
Trying to prevent that has become a top priority. The New York City Police Department, for instance, has one of the most sophisticated local anti-terrorism operations in the country. It has more than 1,000 undercover detectives keeping tabs on members of the Muslim community considered highly radicalized and potentially dangerous.
Security experts say such groups and individuals are even more difficult to detect than traditional al Qaeda cells who report back to Osama Bin Laden, because they are local citizens who may have done nothing to call attention to themselves.
"People who were born in Canada, people who were born in England, don't show up on wanted lists, don't show up on do-not-fly lists, do not have leaders who are known," said Richard Clarke, the former White House counterterrorism czar who is an ABC News consultant. "This is leaderless terrorism. It's self starters. It's cells that are not connected to anything. They are very, very hard to find."
Since such people may be operating on their own, without direction from others overseas, they can do their plotting face-to-face.
"There's nothing in their communications that would indicate this is terrorist communication," Clarke said. "The calls are domestic. They're not going back to Afghanistan. And what's probably being said is the equivalent of, 'Let's all get together at Joe's house.' "
The 17 arrested in Canada allegedly have ties to two men from Georgia who were arrested earlier this spring. After the arrests, the FBI accused the Georgia suspects of gathering information about Washington D.C., oil refineries and military installations.
Tonight, police in the United States are looking for any other connections to the Toronto cell.
The Canadian public safety minister said he's been in touch with U.S. officials to assure them that the 17 men under arrest had no plans to attack any targets in the United States.
Alleged Actions and Ideology, Not Religion
In Toronto today, police officials assured the Islamic community that the 17 people under arrest were targeted because of their alleged actions, not their religion.
"There is no accusation against the Muslim community by law enforcement," Toronto Police Chief Bill Blair said. "Our accusations pertain only to the actions of 17 young men."
Those young men, mostly Muslims of South Asian descent, have been charged with plotting attacks on Canadian targets. Police said they had three tons of fertilizer -- the same material that fueled the 1995 Oklahoma City bombing.
"These are people diverse backgrounds, a variety of backgrounds and ages, certainly inspired by terrorist ideology but operating within their own network," said Stockwell Day, Canada's minister of public safety.
It's a big concern for officials in the United States, where Islam is the fastest growing religion. There are seven million Muslims in America, 2.3 percent of the population. Most of them have no terrorist intentions -- which is also true for millions of Muslims in Canada.
"But those 17 people, using dual-use material available at hardware stores and at farm stores, could have done enormous damage," Clarke said.
ABC News' John Yang and Pierre Thomas reported this story for "World News Tonight."