But intelligence analysts suggest the coordinated attacks against the World Trade Center, the Pentagon, and possibly elsewhere have the mark of Osama bin Laden's terrorist network, Al-Qaeda.
"You would think it would have to be bin Laden behind it, because who else would have the audacity, the conceptual audacity of it," says John Pike of GlobalSecurity.org in Washington, D.C.
"His organization fits the profile. There are very few international terrorist organizations with such skill sets capable of launching such a massive and coordinated attack," says Stratfor analyst Jamie Etheridge, in Austin, Texas.
"His organization incorporates the tactics used
suicide, coordinated attack," she says.
"They probably used trained pilots to ram World Trade Center," she says.
But Steven Aftergood, an intelligence analyst at the Federation of American Scientists cautions: "I would like to forego guessing. I think the evidence should be collected and should speak for itself."
"I think what can be surmised is this was the work of an exceptionally well organized and sophisticated group of people and they must have left footprints in one place or another. This was not bin Laden operating out of his tent."
Warnings About Bin Laden's Group
Al-Qaeda is a loose umbrella association of radical groups and people in countries around the world, suspected of association with previous attacks against U.S. interests, including the 1998 bombings of two U.S. embassies in Africa and the attack on the USS Cole in the Yemeni port of Aden last October.
Bin Laden and others allegedly associated with him were indicted in the embassy attacks.
Over the course of the year, the U.S. State Department has issued a number of alerts pointing specifically to the possibility that agents of Al-Qaedamay be planning an attack against U.S. military or civilian targets.
A June 22 alert cautioned the U.S. government has learned "that American citizens and interests abroad may be at increased risk of a terrorist action from extremist groups."
Most recently, on Friday, Sept. 7, the State Department issued a worldwide alert warning "American citizens may be the target of a terrorist threat from extremist groups with links to Usama Bin Ladin's Al-Qaeda organization."
That report cited information gathered in May, which suggested an attack somewhere was imminent. It warned individuals in Al-Qaeda "have not distinguished between official and civilian targets."
U.S. government facilities overseas have been on a heightened state of alert since the bombing of the Cole.
Possible Warning Last Month
An Arab journalist with access to bin Laden said the Saudi-born dissident had warned three weeks ago that he and his followers would carry out an unprecedented attack on U.S. interests, according to Reuters.
"It is most likely the work of Islamic fundamentalists. Osama bin Laden warned three weeks ago that he would attack American interests in an unprecedented attack, a very big one," Abdel-Bari Atwan, editor of the London-based al-Quds al-Arabi, an Arabic-language weekly news magazine, said.
"Personally, we received information that he planned very, very big attacks against American interests. We received several warnings like this. We did not take it so seriously, preferring to see what would happen before reporting it."
Bin Laden is suspected of residing and running terrorist training camps in Afghanistan.
A spokesman for the ruling Taliban organization in Afghanistan today said he couldn't rule out bin Laden was involved.
Asked, through translation, whether he could rule out bin Laden involvement, spokesman Wakil Ahmed Mutawakel responded: "No, up until now, no one has blamed or accused or him."
"We criticize terrorism in all its forms," he said in a news conference in Kabul, Afghanistan.
It may be most difficult determining the identities of those persons directly responsible, since they died in the suicide attacks.
"It's likely we're probably never going to know who the people were who actually were on board the aircraft, and it will be very difficult to trace who those people were," says Tim Brown, a counterintelligence expert also with GlobalSecurity.com.
Whether bin Laden's group or some other group was behind it all will take, in part, a massive review of intercepted global communications gathered prior to today.
"Unavoidably, they're going to have to go back through everything they've previously collected and see what if anything they missed," says Pike.
"That's like trying to look for the gene that causes Alzheimers," says Brown. "They're going to have to go through tons and tons of data and they may never have even had the tidbit they needed."