Twelve years after al Qaeda slaughtered nearly 3,000 Americans on U.S. soil, the FBI has under watch as many as 100 people inside the homeland suspected of being linked to or inspired by the terror group, intelligence and law enforcement officials told ABC News.
Additionally, intelligence and law enforcement officials had anticipated -- even before April's Boston Marathon bombings -- that this approximate number of terror cases wouldn't change in the years ahead, even with arrests made, because of new cases expected to surface.
Despite years of losses from drone strikes overseas and counter-terrorism operations inside the American homeland, the al Qaeda network still survives thanks in part to its American recruits.
"I think that is the most disturbing thing, to see Americans switching sides and going over to the enemy," Rep. Michael McCaul, R-Texas, Chairman of the House Committee on Homeland Security, told ABC News.
Some of the Americans that have gone over to al Qaeda have risen far enough in the ranks that in the years after the Twin Towers fell, often the public voice of the perpetrators of that horrible attack speak with an American accent.
"America is absolutely awash with easily obtainable firearms," said Adam Yahiye Gadahn, a California Muslim convert, in a 2011 Al Qaeda video urging individual violent jihad. "So what are you waiting for?"
Gadahn, who once tore up his U.S. passport on camera, is now in Al Qaeda's leadership in Pakistan, regularly producing videos in English and Arabic. He is the first American since the 1950's to be charged with treason, indicted in 2006.
Gadahn is one of five Americans the U.S. has offered a total of $21 million in rewards to help capture because they served under Osama Bin Laden or his henchmen. The five, however, are only a fraction of the number of Americans believed to be fighting for al Qaeda or one of its affiliates.
Americans taunting their own countrymen -- or luring them into the fight -- is a new and troubling reality about the resilience of al Qaeda even after the killings of Osama bin Laden and Yemeni-American al Qaeda cleric and leader Anwar al-Awlaki two years ago.
The accused American terrorists come from small towns and big cities, law enforcement officials told ABC News. They include a man who grew up on Monte Vista Road in Phoenix, 30-year-old U.S. Army veteran Eric Harroun.
This year he became one of about a dozen Americans who authorities say are fighting in Syria with a group that has sworn allegiance to al Qaeda, called Jabhat al-Nusra.
Harroun, who was lured out of the region by the FBI and charged with terrorism in a Virginia federal court, allegedly posted videos on Facebook of his adventures in Syria with fellow fighters, including one where he addressed Syria's president Bashar al-Assad, saying, "Where you go we will find out and kill you. Do you understand?"
Eric Harroun's father says his son just fell in with the wrong people.
"He's not any terrorist, not any more than I am," Darryl Harroun said in an interview today with ABC News from Phoenix.
The younger Harroun is expected to go to trial in two months on terror charges.
American recruits to al Qaeda are also showing up in other hot spots across the Middle East and Africa, intelligence sources said.
"As an American citizen, I'm shocked. I'm amazed that something like that occurs," Shawn Henry, who retired last year as a senior FBI official, told ABC News.
Henry, now an executive at the cyber security firm CrowdStrike, said Americans or "U.S. Persons" -- non-citizens who have lived here and have certain legal rights -- radicalized to violent Islamist extremism are "a minority," but the FBI's highest counterterrorism priority.
"Once they get that into their blood, it's a threat," Henry said.
At least 50 young American men have been tracked to the al Qaeda group fighting in Somalia, al-Shabaab, where a young man from the small town of Daphne, Alabama, Omar Hammami, became a top commander.
"Our main objective, one of the things we seek for in this life of ours, is to die as martyrs," Hammami explained in one video, among numerous he made to help al-Shabaab draw Westerners into the fight.
More than 15 U.S. citizens have been killed fighting with al-Shabaab, and at least 20 remain unaccounted for in Somalia.
A new American voice in the Somali terror organization, the as yet unidentified "Abu Ahmed al-Amriki," appeared in a February video, said his countrymen should fight Western governments in Afghanistan, Somalia and Mali.
"America is going down and the Caliphate is rising," he said, brandishing an AK-47.
Hammami -- who remains committed to violent jihad despite a falling out with al-Shabaab leaders -- is now one of the five Americans with U.S. rewards ranging from $1 to $5 million on their head because of their alleged Al Qaeda leadership positions. Others are men from Waukesha, Wisconsin, Brooklyn, New York and Buffalo, New York.
For security reasons, the details of those 100 or so individuals under surveillance inside the U.S. are closely guarded, and both U.S. and European officials say they're focused on Westerners joining al Qaeda in places such as Syria, which is relatively easy to enter.
Asked via Twitter last March about those like him, who turn against America by joining al Qaeda affiliates such as al-Shabaab in Somalia, Hammami did not deny the threat they pose.
Somalia has "many muhajirs from U.S. And dangerous. True," tweeted @abumamerican, an account believed by U.S. officials to be Hammami's.