Counterterror sources tell NBC News that some of those arrested in London recently traveled to Pakistan, and, while there, are believed to have trained in explosives.
These sources also say money was wired from Pakistan to London, presumably to purchase plane tickets. Two men also were arrested recently in Pakistan in connection with the plot.
"Whenever there's a Pakistani component to these type of operations, that normally means there's an al-Qaida element as well," says NBC terrorism analyst Roger Cressey.
NBC News has learned that at least of the London plotters made a martyrdom video to be aired after the suicide attacks — an al-Qaida trademark.
And then there is the plot itself — strikingly similar to one foiled in the Phillipines 11 years ago. That plot — called Operation Bojinka — was to bomb 12 U.S. airliners as they crossed the Pacific Ocean, using liquid explosives. Today's plot repeats both targets and methods.
"It is certainly true that al-Qaida studies past terrorist plots that have failed and attempts to recreate them in a more successful environment," says NBC terrorism analyst Evan Kohlman.
When the first attack on the World Trade Center failed, al-Qaida attacked again, on 9/11.
Also, there are al-Qaida's messages. Both Osama bin Laden and his No. 2, Ayman al-Zawahiri, have promised something big as recently as last month.
"This was at least a component, if not the entirety, of al-Qaida's plans and proclaimed announcements of an upcoming spectacular attack," says Mark Marshon, assistant director of the FBI.
While the extent of al-Qaida's involvement is not yet clear, some experts do see fingerprints.
"You'll find al-Qaida support in the bomb-making, in the training of individuals, in the movement of money and perhaps even recruitment," says Cressey.
If this is indeed the work of al-Qaida, many counterterror experts see it as proof that al-Qaida has been able to reconstitute itself, despite heavy losses since 9/11.