MORE ON THIS STORY FULL COVERAGE • America Attacked VIDEO • Who Is Osama bin Laden? • World Leaders React to the Attack • Complete Video Coverage of the Attacks IN DETAIL • Interactive: Terror Strikes America RELATED STORIES • Index: A World of Terror • Violent Influence of Afghan War Vets

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"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.

"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 [attack], coordinated attack," she says. "They probably used trained pilots to ram the World Trade Center."

Sen. Orrin Hatch, R-Utah, ranking member of the Senate Judiciary Committee, told ABCNEWS that "top people at the CIA" told him "just about everything points in the direction of Osama bin Laden." Hatch also said U.S. officials had some data suggesting bin Laden associates were on at least one of the aircraft that crashed apparently was brought down today.

Steven Aftergood, an intelligence analyst with the Federation of American Scientists, earlier cautioned: "I would like to forgo 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," he adds. "This was not bin Laden operating out of his tent."

Secretary of Defense Donald Rumsfeld in a press briefing today declined to discuss at this time whether bin Laden is suspected.

Thousands of FBI agents are investigating, Their probe will include visits to Boston's Logan and Virginia's Dulles international airports, from which three of the four planes that crashed in apparent hijackings today originated. They will be dissecting passenger lists for clues as to who may have been responsible, and calling relatives of the people on the lists. Agents also are executing search warrants based on the flight manifests.

The New York field division is deploying its Evidence Response Team to the World Trade Center to gather evidence.

Warnings About bin Laden’s Group

Created in 1998, al Qaeda is a loose umbrella association of radical groups and people believed to operate in dozens of countries around the world and 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 to the possibility that agents of al Qaeda may be planning an attack against U.S. military or civilian targets.

A June 22 alert said the U.S. government had 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 [Osama bin Laden's] al Qaeda organization."

That report cited information gathered in May that suggested an attack somewhere was imminent. It warned that individuals in al Qaeda "have not distinguished between official and civilian targets."

U.S. officials familiar with terrorist investigations tell ABCNEWS the majority of threats the United States has detected in recent days and weeks have been nonspecific and concerned overseas targets.

They said they were focusing on gathering information that would lead to the perpetrators and that they believe there was information pointing on the direction of bin Laden's organization.

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 news agency.

"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," said Abdel-Bari Atwan, editor of the London-based al-Quds al-Arabi, an Arabic-language weekly news magazine.

"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 war-ravaged Afghanistan.

A spokesman for the Taliban, the hard-line Islamic organization that rules most of Afghanistan, said today he couldn't rule out bin Laden was involved.

Asked through a translator whether he could rule out bin Laden, 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.

Independent Cells?

Analysts say that, in a new trend, independent terrorist cells loosely associated with bin Laden have begun to focus on the United States as a target.

Clues about such groups were given to U.S. authorities by convicted Algerian bomb smuggler Ahmed Ressam, who was arrested in Washington state in December 1999 and has confessed to planning to explode a large bomb at Los Angeles International Airport.

Ressam, who authorities believe was linked to bin Laden, told authorities the exiled Saudi dissident has been seeking in recent years to join forces with other terrorist groups, including battle-hardened Algerians who were veterans of that country's brutal civil war.

Ressam's testimony was "very chilling and dramatic testimony about his training in camps in Afghanistan and later he was sent as a secret cell into Canada and turned on then during the millennium [celebration as 1999 ended and 2000 begain]," says William Livingstone, deputy communications director for the corporate security firm GlobalOptions. "He was saying that there are many different secret cells that are trained and deployed around the world."

An article in the respected Jane's Intelligence Review last month said cells have been detected in more than 50 countries — including the United States, Canada, Italy and Germany, where they were neutralized but "have since been replaced."

CIA Director George Tenet in February noted the trend in an annual statement on worldwide threats before the Senate Select Intelligence Committee.

Bin Laden's "organization is continuing to place emphasis on developing surrogates to carry out attacks in an effort to avoid detection, blame, and retaliation," Tenet said. "As a result it is often difficult to attribute terrorist incidents to his group."

Rogue Terrorists

Terrorists not closely connected to another group are also a concern to for U.S. officials.

The classic example is the Middle Eastern-born Ramzi Yousef, who was convicted for masterminding the February 1993 bombing of the World Trade Center, which killed six people. Yousef also was found to have plotted to blow up 11 U.S. airliners in an attack coordinated with at least five other men.

Such "loosely affiliated extremists may pose the most urgent threat to the United States at this time because their membership is relatively unknown to law enforcement, and because they can exploit the mobility that emerging technology and a loose organizational structure offer," former FBI director Louis Freeh told a congressional committee in 1998.

The Hunt

It may be most difficult determining the identities of those persons directly responsible for today's attacks, 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.

Determining 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 Alzheimer's," 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."

ABCNEWS' Howard Rosenberg contributed to this report.