Strong concerns that terrorist teams in Europe have selected their targets, completed their surveillance, eluded capture and are now ready to strike at airports and tourist attractions have prompted the State Department to ready a highly unusual travel advisory for Europe, multiple law enforcement and intelligence sources tell ABC News.
Intelligence and law enforcement officials have information that the teams could at any time launch a "Mumbai style" terror attack that targets civilians for death or hostage taking. The 2008 Mumbai attack used small arms and explosives to kill 175 people and paralyze the Indian city for days.
The current concerns are for scenarios that include opening fire at airports in Europe as well as executing similar attacks at "soft" targets like tourist attractions or hotels.
According to ABC News sources, the terror plotters have moved through the surveillance stage, checked back in with al Qaeda in Pakistan, and have received the go-ahead to strike.
Officials said earlier that Osama bin Laden had approved or blessed the attack plan.
The suspects reportedly include British and German citizens, who may be of Pakistani or Afghan ethnicity.
ABC first reported the threat of Mumbai-style attacks last week. Since then senior intelligence and law enforcement officials have continued high-level meetings to assess the intelligence and weigh the appropriate additional responses.
By Thursday afternoon the unusual travel alert became a topic for discussion. Officials say that when it comes, the alert is expected to be carefully and vaguely worded.
European and U.S. authorities first learned of the plot over the summer following the capture of a suspected German terrorist who had been training in Pakistan.
Intensified drone strikes in Pakistan have been confirmed as in part aimed at paralyzing the attackers.
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.
As the threat picture continues to unfold it appears to include the possibility of multiple coordinated attacks in multiple countries. As such, it would be the most significant al Qaeda plot to have reached the operational stage in recent years.
The Department of Homeland Security (DHS) had no official comment, but sources say no change in the U.S. threat level is expected unless specific information regarding a U.S. threat develops.