Recent law enforcement operations within the United States have helped to flush out chatter that added to earlier concerns about the U.S. homeland as a possible additional target of the attacks.
Known targets are believed to include England, France and Germany. Additional European countries, including Italy and Belgium, are also targets, multiple sources say.
Siddiqui's claims about a multi-city plot against Europe have been bolstered by other "highly reliable" sources of information, US and German intelligence officials said Thursday.
Siddiqui told American interrogators at the Bagram Airfield in Afghanistan that Osama bin Laden had personally blessed the plan, officials said.
Since then, US and German officials said, Siddiqui's claims have been verified by a second captured German terror recruit and "other sources" that officials were reluctant to describe in detail for fear of compromising law enforcement operations.
"There are several different sources, all confirming that there are plots afoot by al Qaeda central, that is to say the Osama bin Laden organization in Pakistan, to do attacks in Europe," said Dick Clarke, a former White House national security official and now an ABC News consultant. "Now they don't have anything that points to the United States yet, but if there were to be simultaneous attacks in Europe, it's at least possible there would be a simultaneous attack in the United States as well."
German intelligence officials told ABCNews.com that about 45 other "potentially dangerous" individuals in Germany are being tracked as officials seek to prevent an attack from taking place.
Siddiqui worked as a cleaner at the Hamburg airport and was a "devout member" of the al-Quds mosque in Hamburg, where Mohammed Atta and other 9/11 hijackers gathered prior to their attack.
The mosque was closed after Siddiqui's arrest, based on the information he provided. Officials told ABC News that Siddiqui had been under surveillance since 1997 and left the country for Pakistan in 2009.
Officials in Germany and the United States said they still did not have a specific date of the timing of the plot, and US officials say they believe the plot's "trigger date" may have been delayed because of the media coverage and government travel alerts.
"What we've seen in the past with al Qaeda, when one of their attacks is partially exposed before it happens, they pull back, they regroup, they wait a while," said Clarke. "Sometimes they wait as much as one year. In the case of 9/11, I believe that they waited perhaps a month or two from when they originally intended to do it because of the terror alerts that we conducted back then. They know it's better not to stage these things when everyone's looking for them."