This is part of the America's Future series airing on FOX News Channel, looking at the challenges facing the country in the 21st century.

WASHINGTON — Many Americans believe that the threat of homegrown terrorism is gravest in Europe, but according to the U.S. government, in the last 18 months more than a half-dozen plots were thwarted right here at home.

Sen. Joe Lieberman says the threat is growing.

"Bottom line: There is a threat of homegrown terror in America," he said. "It is not as great as in European cities, but it is growing and we have to take it seriously."

Three young Ohioans were convicted in June on terrorism charges after officials gathered evidence against them that included suicide bomb belts filled with ball bearings.

Mohammed Amawi, Marwan el-Hindi and Wassim Mazloum were part of a terror cell in Toledo that wanted to launch attacks against U.S. troops overseas — made all the easier by their status as Americans.

"Being an American gives you a passport around the world," said Andrew Cochran, chairman of the Counterterrorism Foundation and editor of the Counterterrorism blog. "These again are instances of homegrowns who take it to the limit. ... These people wanted to go all the way to Iraq."

Other homegrown terrorists have planned attacks on the homefront. Derrick Shareef, then just 22, was inspired by a violent Islamist ideology to plan a grenade attack against a shopping mall in Rockford, Ill. He eventually pleaded guilty to terror charges.

The Internet is fast becoming the dominant tool for the training and recruitment of terrorists. Some lawmakers are attempting to shut down such sites and those with the most extreme propaganda tapes, often made by Al Qaeda's media arm, As-Sahab.

"I am continuing to work to try to bring down the terrorist Web sites on the Internet," Lieberman said. "I think the critical role… [is] reaching out to try to stop the problem in local areas before it starts."

The shutting down of certain Web sites is a prospect some critics are dreading.

"We have a First Amendment and we champion the Constitution, and so in no way, shape or form should we engage in censorship of the internet," said Kareem Shora, national executive director for the American-Arab Anti-Discrimination Committee.

"But the community recognizes that the Internet can be a dangerous tool for youth," he said, "so we're working on that by giving constructive alternatives to the youth."

Ultimately, experts say, the Web will replace traditional terrorist training with cyber training, making it easier to recruit homegrown terrorists.

"It's not a substitute for physical field training, but it can come close to creating situations on how to train for an urban attack, a mall attack," Cochran said. "It's somewhat like what some of the 9/11 hijackers used in flight simulation software."

According to experts, young, middle-class American Muslims are most at risk — men who don't know a lot about their religion and in an effort to educate themselves fall victim to an extreme ideology.

Yet American Muslim groups say that formula amounts to racial profiling.

"Giving parameters as far as race, religious views or age groups really misses the point. We should be much more sophisticated in the way we approach threats against our country," Shora said.

U.S. lawmakers also are looking at ways of addressing the root causes of homegrown terrorism.

"We also have to ... reach out and grab the hearts and minds, particularly of young Muslim-American males," Lieberman said "We've got to count on their family members and close friends if they see them heading in this direction."

Click here to see more reports on America's Future.