Intelligence and counterterrorism officials on Thursday said the scale and sophistication of the alleged scheme to blow up jetliners over the Atlantic could mean that al-Qaida, whose central command has been severely damaged since 2001, is again able to direct attacks.

But some specialists on the shifting networks of international terrorism said the alternative explanation — that homegrown British jihadists had conceived a plot of such ambition — might hold even graver implications for the future.

"The great problem is that al-Qaida has moved far beyond being a terrorist organization to being almost a state of mind," said Simon Reeve, author of a 1999 book on Osama bin Laden and his associates. "That's terribly significant, because it gives the movement a scope and longevity it didn't have before 9/11."

FBI Director Robert S. Mueller III said the scope and targets of the thwarted plot were "suggestive of al-Qaida direction and planning," and other top officials said the plan reflected the terrorist network's penchant for spectacular and simultaneous assaults.

The suspects arrested in Britain are all British citizens, primarily of Pakistani descent, and with possible ties to Pakistan, according to British officials. Bin Laden and his deputy, Ayman al-Zawahiri, are believed to be in hiding in Pakistan, which counterterrorism officials fear has become a center of terrorist plotting.

John O. Brennan, former director of the National Counterterrorism Center, said Qaida connections for the alleged plotters could cover a range of possibilities, from direct ties to the group's leaders to links with people who may have trained in a Qaida camp at some point. The latter was more likely, he said.

Whether it was hatched in Britain or Pakistan, Brennan said, "the intention of al-Qaida was to create a base or foundation for a long-term struggle. Its leaders are thinking in terms of the Crusades and a conflict that lasts for many, many years."

Most counterterrorism officials said that neither the Madrid, Spain, nor the London attacks were directed by al-Qaida, though some officials believe one or more of the participants may have had contact with Qaida-trained operatives.

If the latest plot can be traced to bin Laden's direction, rather than his inspiration, that would be an unwelcome surprise, Reeve said. "That means he's still capable of directing a major attack," he said. But if bin Laden was a bystander, Reeve said, "that could be worse news," suggesting a "fragmentation of the terrorist threat."