Heading into the tenth anniversary weekend of the September, 11 attacks, the nation’s top counterterrorism officials have ramped up security measures, and are looking out for a “lone wolf.”

Homeland Secretary Janet Napolitano told ABC News that “we don’t right now have intelligence that a big plot is in the works.”

But while there is no known specific plot by Al Qaeda or other terrorist groups, Napolitano warned, “now that differentiates from the lone wolf, the lone actor that we may not know about, who may already be in the United States and so it requires us to be vigilant and the public to be vigilant. ”

In an exclusive, wide-ranging interview with ABC Senior Justice correspondent Pierre Thomas, Napolitano recalled where she was on September 11, 2001, and surveyed the state of U.S. defenses against Al Qaeda.

While “core al Qaeda” in Pakistan and Afghanistan has deteriorated, she said, Al Qaeda on the Arabian Peninsula has grown into a real threat.

A big reason al Qaeda is so potent, she said, was its leader, Anwar Alwaki.  Alwaki is an American Imam who became radicalized and is now operating his terror group in Yemen.  Sources have told ABC Alwaki is intent on striking the United States in any way he can.

In fact, an ABC News analysis shows the Alwaki is either behind, or inspired, 19 Americans who federal prosecutors say were homegrown radicals. Thomas asked the DHS Secretary, “Do you share the concern that he maybe the number one threat out there? ”

“He is at the top of the list, if not at the top,” Napolitano answered. “He knows Western ways, he kind of knows how to market to Westerners and we know that Al Qaeda is trying to recruit.

Napolitano said Alwaki is using online magazine and videos to try to lure disaffected Americans into his violent ideology.

“You know the internet is a powerful tool for good, for friendships, for commerce, for what have you, but it also can be used for evil,” the DHS Secretary said.  “And we see it being used to recruit young Americans, not necessarily even young Americans, to a terrorist-type ideology.”

She added that one of the biggest changes she has seen as DHS Secretary, “is he movement toward the home-grown violent extremist. The person who for whatever reason decides to attack his fellow citizens.”

To combat the surge in homegrown terror, Napolitano said, “requires us to focus more on training local law enforcement, they’re the eyes out there.”  And she emphasized, getting the public engaged, through the “See something, say something” program, is key to stopping homegrown threats.  “The public,” she said, “are our other set of eyes.”

On September 11, 2001, Napolitano was the Attorney General of Arizona. She said when she realized the nation was under attack, she called the governor’s office to consult on how to protect the critical sites in her own state.

“What’s interesting was that there was really no playbook to use, I mean we didn’t know what was expected in terms of closing down airspace,” Napolitano said.  ”we had a nuclear power plant you know twenty miles or so outside of Phoenix, we had to deal with that, and what would happen with the borders, and we didn’t really have a connect into the Federal government to answer those questions.”

Today, she says, she has helped make sure that officials know what to do in case of a terror attack. “All of those things exist now, those kind of playbooks: who do you call, what do you do, that’s all in place now.”

How does she keep focused on preventing another attack?  Napolitano told us she keeps a reminder of September 11 close. “I have a piece of the towers on my desk.”