After more than a decade hunting Al Qaeda, U.S. counterterrorism agencies are reviewing their understanding of the terror group, which they strongly believe was behind the plot to blow up some 10 bomb jetliners bound for the U.S.

Several Western government officials, who spoke on condition of anonymity about the ongoing investigations, said they don't yet have hard evidence to pin the attack on Al Qaeda's central leadership — Usama bin Laden, his deputy Ayman al-Zawhari or a tier of key operatives below them.

But the plot bears all of the best-known earmarks of the terrorist group.

More than two dozen radicals, broken up into two- or three-member teams, were to assemble bombs made from everyday items on as many as 10 airplanes and blow them up as they traveled from Britain to the United States.

One intelligence official said not all the attackers knew one another, following operational security lessons taught in Al Qeida training manuals.

Among other Al Qaeda trademarks:

— The plan was complex and had a carefully timed sequence for maximum dramatic impact.

— Financial and training support came from Pakistan, a hub for extremists where at least two of plot participants traveled.

"Certainly in terms of the complexity, the sophistication, the international dimension and the number of people involved, this plot has the hallmarks of an Al Qaeda-type plot," Homeland Security Secretary Michael Chertoff said Friday.

An intelligence official said that investigators are focusing on whether one of the people arrested early in the Pakistan investigation was an operational planner. The official said Rashid Rauf, who has also been identified by Pakistanis as a key player, spent a lot of time overseeing the plotters and providing instructions to them.

Pakistan is questioning at least 17 people, including Rauf and one other British national whose name has not been released. Authorities in Pakistan believe they have nabbed the main players in the plot, but say two or three people are still at large.

Because of Rauf's suspected ties to Al Qaeda figures, some in government believe there is a high probability the group will be connected to the plot, the intelligence official said. It was not clear how cooperative he has been with interrogators.

Some agencies have also focused on an Al Qaeda explosives expert in Pakistan named Matuir Rehman, but his involvement is not certain and a matter of debate within the U.S. government.

Counterterrorism officials are particularly reviewing the plotters' connections to Pakistan, where Al Qaeda has a substantial base. An intelligence agency recently intercepted a message from that country to the plotters in Britain urging them to move forward, after arrests were made in Pakistan, said a Western government official who spoke on condition of anonymity.

Bin Laden is believed to be hiding in the region along the Afghan-Pakistan border.

The disrupted plot has intelligence analysts taking a second look at questions such as what motivates citizens of a democratic country who are under age 25 to become radicalized, then leave their home country for terrorist training and return.

An important question to improve the understanding of terror groups, the official said: "How do they have, at that age, that level of training and expertise and everything to carry out such a sophisticated effort?"

It's not yet clear why the young radicals designed the latest attack plan, but officials suspect it may be based on a foiled plot from the mid-1990s. Then, Al Qaeda was planning to blow up about a dozen airplanes on trans-Pacific flights, but a fire in a Manila apartment foiled them.

Chertoff noted the similarity, calling it "certainly reminiscent" of a plot devised by senior Al Qaeda figure, Khalid Sheikh Mohammed, who was also behind the Sept. 11, 2001 attack on the United States.

"Very sophisticated, the idea of multiple terrorist attacks at the same time," he said Sunday on ABC's "This Week." "When you put those pieces together it is certainly the scope and scale that is comparable to the worst that Al Qaeda has done."

Al Qaeda has returned to attacks that don't go off as planned. The 1993 World Trade Center bombing killed six, rather than the hundreds or thousands that Al Qaeda normally targets. The group returned to the target in 2001, with devastating effects.

The 2000 attack on the USS Cole was preceded by an attempted strike on the USS The Sullivans, but the attack boat sank because it was loaded with too many explosives.

CIA Director Michael Hayden told an audience this spring that Al Qaeda and its affiliates "still pose the greatest threat to the homeland and US interests abroad" of any single terrorist group.

Yet he and his two predecessors — George Tenet and Porter Goss — have warned the threat from Al Qaeda is evolving from an organization based in Afghanistan to an amorphous ideology with a wider geographic reach.

A former intelligence official, who spoke on condition of anonymity, said the surprising aspect of the latest attack plan was the size of the operation.

The number of people involved, its sophistication and its scope indicate extremist networks still have substantial capabilities, the official said.

"To get that many people to take part in something like this ... that are not wannabes or people who are just copycats or looking up how to do this on the Wikipedia, there must be some more sophisticated links back to central Al Qaeda," the official said.