Its scope was ruthlessly ambitious, causing destruction officials say would have been "unimaginable." The alleged plot to take down several U.S.-bound planes with liquid explosives appears to be unlike anything the world has seen in years.

Counterterrorism officials said Thursday the London plot appears to bear the fingerprints of al-Qaida, and may have been "the Big One" they have been dreading since Sept. 11, 2001, particularly as the five-year anniversary of the carnage approaches.

More than 20 people have been jailed, terror threat levels have been raised to some of their highest levels, and hundreds of flights have been canceled worldwide.

"The scope or the magnitude of this attack is much larger than previous attacks," said Rohan Gunaratna, a terrorism expert at Singapore's Institute of Defense and Strategic Studies.

He added that everything known so far points to involvement by Osama bin Laden's terror group.

"It is a classic al-Qaida tactic. It is a hallmark of al-Qaida to carry out coordinated, simultaneous attacks, and the aviation domain is certainly known to al-Qaida. They have obvious experience in working around that system and extensive knowledge of the aviation domain."

U.S. Homeland Security Secretary Michael Chertoff echoed those sentiments, saying the attack "was sophisticated, it had a lot of members and it was international in scope." He added that: "It was in some respects suggestive of an al Qaida plot," but cautioned that the investigation was still under way.

There have been dozens of thwarted plots around the world since the Sept. 11 attacks, and several that were murderously successful. Suicide bombers killed 52 people in London on July 7, 2005, 58 in two attacks in Istanbul, Turkey, in 2003, and 202 in Bali in 2002. Islamic radicals killed 191 people in Madrid on Mar. 11, 2004, then blew themselves up days later when police were closing in.

While al-Qaida's call for global jihad clearly acted as inspiration, there has been no direct evidence that bin Laden or his No. 2, Ayman al-Zawahri, had advance knowledge of those attacks, that they helped plan them, or that they provided financial or logistical help to those who carried them out.

The group's failure to match the destruction it inflicted in the Sept. 11 attacks has led to speculation that a global dragnet that has forced bin Laden into hiding and ensnared many of his most trusted deputies may have degraded al-Qaida's abilities.

Analysts said Thursday that is a theory to be believed only at the world's peril.

The airline plan had the potential to dwarf the attacks of recent years — killing hundreds, perhaps thousands.