Terrorism experts quickly arrived at a clear conclusion one year ago: Only one man had the motive and capability to carry out those searing attacks on New York and Washington.
Today, no one is certain if that man Osama bin Laden is dead or alive. But one thing is sure: his Al Qaeda network has not successfully launched another catastrophic attack on the US.
"The war on terror and the damage done to Al Qaeda has at least blunted that threat," says Bruce Hoffman, a terrorism specialist at the RAND Corp. "And that's a significant achievement."
Still, Al Qaeda is hardly impotent. In fact, many experts warn that its far-flung cells are still capable of pulling off attacks. But the absence of another spectacular strike so far is a sign of numerous small successes the US has scored in its war on terror.
Over the past year, experts point out that the US has:
Evicted the Taliban regime from Afghanistan, depriving Al Qaeda of its training camps and operational headquarters.
Detained or arrested one-third of Al Qaeda's leadership.
Established cooperative relationships with 100 countries, where some 3,000 individuals have been detained or arrested.
Blocked some $112 million in Al Qaeda funds.
Retrieved large amounts of information, including instructional manuals, videos, diaries, CD ROMS, and hard drives from suspected terrorists.
Still, intelligence officials inside and outside government say the war on terror remains in its infancy. For starters, Mr. bin Laden may still be out there, along with his No. 2, Ayman al-Zawahiri, and the remaining two-thirds of his leadership. And experts know cells of his network are located in many other countries, including the US.
Speculation is surfacing again that bin Laden may be dead. "If he's not, he's the most prudent man on earth," says one official.
That's because intelligence agencies have not been able to intercept his voice in nearly a year. They've also noticed his bodyguards have scattered to different parts of the globe. They believe the only reason that would happen is if their leader were no longer around.
"He's not someone who has enjoyed being a wallflower," says a former CIA counterterrorism analyst. "If he were out there, undefeated, he would be broadcasting defiance through some method.... It's not like him to shut up."
Until there's proof he is dead, however, the US seems intent on continuing the hunt. "With the lack of evidence indicating he is dead, the operating assumption is that he is alive and in the tribal area along the Afghanistan-Pakistan border," says a US government official.
Authorities are also uncertain how much the measures taken to block funds have affected Al Qaeda's finances. Reports earlier this week indicated that Al Qaeda shipped a large quantity of gold from Pakistan to Sudan.
Sheila Bair, former assistant secretary for financial institutions at the US Department of Treasury, admits that is possible. "We don't know how much money is out there, and we may not have identified all their avenues for transferring money."
In the complex world of international finance, US officials realistically are looking at containment rather than closure of the Al Qaeda bank vault. "We can't make it impossible [for them to operate], but we can make it much more difficult," says Ms. Bair.
As the war on terror goes on, it's becoming more difficult for the US to carry out many of its counterintelligence operations, such as eavesdropping and following the money trail. Part of that stems from the success of the Afghan campaign: It scattered and drove the militants underground.
"Going back to 1993, when we began to focus on them, they didn't know we were watching them," says the former CIA terrorism analyst. "Now they know we're completely focused on them. They've tightened their security stopped using laptops, websites, money transfers online."
Perhaps most worrisome is that at least two-thirds of the Al Qaeda leadership and untold numbers of foot soldiers are still out there. And even if bin Laden were dead, the organization could probably function effectively.
Al Qaeda is a multitiered network. While the leadership may have planned the more catastrophic attacks like the 9/11 attacks and 1998 bombings of the US embassies in Africa specialists say there are plenty of lieutenants who can inflict damage.
"Like any good CEO, bin Laden thought about corporation succession at all levels," says the RAND's Hoffman. "He had people who could step in and fill the shoes of the people [who have been killed or captured]."
The US has been successful in foiling some plots, including ones in Paris, Bosnia, Italy, Albania, and Singapore. The question is where and when the next strike might come from here.
US officials profess to be getting valuable information out of some of the suspected terrorists in custody. But there are limits to the amount of human intelligence that can be gathered.
"That information has led us to other individuals and given us ideas of what their intentions and long-term plans are," says one government official. "But there are still a lot of other important players out there who have the capability of planning and carrying out attacks."