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Nearly a decade ago, a group of Saudis and other men from the Middle East came to the United States to carry out the worst terrorist attack on the U.S.

Not a single one had American citizenship.

Almost nine years after the September 11 attacks, the threat of another major terror strike is still a concern, but where the threat is coming from has changed.

A growing number of American citizens and longtime residents of the United States are becoming radicalized enough by al Qaeda's extremist ideology to kill their fellow Americans, counterterrorism officials say.

A growing number are also learning the bomb-making skills necessary to become potentially dangerous terrorists, the officials say. They are training in the mountains of Waziristan in northwestern Pakistan, where al Qaeda still enjoys significant safety.

That's where, according to the U.S. government, alleged Times Square bomber Faisal Shahzad was trained by the Pakistani Taliban, a group with close ties to al Qaeda.

Shahzad's case has strong similarities to that of another American who plotted with terrorist groups in Pakistan to attack the United States. His name is Bryant Neal Vinas, a Catholic convert to Islam from Long Island, New York, who became radicalized, traveled to Pakistan to join up with al Qaeda and helped Osama bin Laden's terrorist organization plot a bomb attack on New York City.

When news of Vinas' arrest broke last summer, family members, friends and terrorism experts where dumbfounded by how a studious, middle-class, baseball-loving, all-American kid and onetime U.S. Army recruit could end up plotting to kill in the name of al Qaeda.

CNN's investigation into Vinas has resulted in an intimate portrait of a homegrown terrorist, charting the disturbing story of a young American's obsessive quest to join al Qaeda.

Vinas' case sheds significant light on why Shahzad and an increasing number of other young Americans have become seduced by al Qaeda's ideology.

Both Vinas and Shahzad were well-integrated into American life before becoming radicalized. Both traveled to the heart of al Qaeda's operational command in Pakistan's tribal region along the border with Afghanistan.

And both allegedly met with the most senior leaders of the Pakistani Taliban in the weeks before allegedly plotting against the United States.

"Bryant Neal Vinas is almost a poster child for the process, the unremarkable nature of the people who might go through this process and, frankly, the potential to link up to al Qaeda and the danger that presents," according to Mitch Silber, the director of intelligence analysis for the New York City Police Department.

Several top U.S. counterterrorism officials had the same message: Americans radicalized at home and trained in Pakistan represent a new and disturbing threat to the American homeland.

The changing face of terrorism

"In the 9/11 world and in the immediate aftermath, the theory was and the reality was that a terrorist attack, if it were to occur again on U.S. soil, would be someone coming from abroad and coming in to the United States," Homeland Security Secretary Janet Napolitano said. "That paradigm has changed, and there are now individuals in the United States, some who have grown up here and are American citizens. ... They haven't done anything to violate the law, but yet they have become radicalized to the point of violent extremism and to the point of ... considering coming back to the homeland and conducting an attack of some sort."

In the last year, there have been 16 cases of Americans or American residents implicated in Islamist terrorism, a surge in such cases. The Times Square plot is case No. 17.

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