This story has been updated.
U.S. authorities plan a law enforcement surge this week along Amtrak routes, an exercise called RailSafe, and the heads of the country's biggest mass transit systems were briefed today on the possible terror threat, all part of what is being called an abundance of caution.
Amtrak is holding a high-security exercise Friday in which uniformed officers will be a visible presence on national transit routes. RailSafe will include all the local police agencies along the Amtrak routes involved in the exercise.
"If al Qaeda is planning simultaneous attacks in Europe," said Richard Clarke, former White House national security official and now an ABC News consultant, "there's nothing to say they could not also include the US on that list of simultaneous attacks."
A senior DHS official said the rail exercise is "long-planned" and "is not connected in any way" to the terror threat in Europe.
The stepped-up security comes as the French arrested 12 terror suspects in Bordeaux and Marseilles, and as the U.S. used CIA drones to attack a suspected center of the plot in Pakistan.
The target hit Monday was one of the terror training camps in the Waziristan region where U.S. officials say a contingent of German citizens of Afghan and Turkish descent have been preparing for jihad against Europe.
U.S. officials say some have already been dispatched, likely those with their faces obscured in a recently released propaganda tape.
But Pakistani officials told ABC News that at least five people were killed Monday at a terror training camp where German citizens have been recruited for the alleged European plot.
The strike came a day after the State Department issued a highly unusual travel advisory for Americans going to Europe because of the potential threat of Mumbai-style commando attacks on civilians, possibly by terrorists of German origin based in Waziristan. Authorities learned of the possible plot this summer from a German national who had been training for jihad and is being held by the U.S. in Afghanistan.
Terror Cell In Hamburg
In an interview Sunday, Pakistan's Ambassador Husain Haqqani told ABC News that the plot's leaders had been identified and targeted.
"I think that several people who were involved in the plotting have been targeted, and the others are certainly on the radar of U.S., Pakistani and European intelligence services," Haqqani said.
Among the possible targets in the suspected European terror plot are pre-security areas in at least five major European airports, a law enforcement official told ABC News. Authorities believe terror teams are preparing to mount a commando-like attack featuring small units and small firearms modeled after the Mumbai attack two years ago that killed 175.
U.S. authorities say they have tracked one of the suspected German terror cells to the German city of Hamburg.
One suspect now in custody worked as a cleaner at the Hamburg airport, and others still being sought attended the same mosque in Hamburg where the 9/11 hijackers gathered.
To the amazement of US officials, it turns out the leader, the imam, of the mosque is the same man accused by the U.S. nine years ago of helping finance the 9/11 plot, Mamoun Darkazanli.
"The mosque went back to being a very radical place where people are recruited for attacks where attacks are discussed," said former White House national security official Richard Clarke, now an ABC News consultant, "and German intelligence apparently stopped looking closely at the mosque where a lot of 9/11 was planned."
Darkazanli, who was never charged by the Germans, declined to comment about the latest plot when approached by ABC News.
In France, the Paris prosecutor's office, which is in charge of terrorism cases, has confirmed that 12 people were arrested in two separate and unrelated operations.
Two men were arrested in Marseilles and a third in Bordeaux. The three are suspected of links to Al Qaeda, and of connections to a Frenchman of Algerian origin recently arrested in Italy with explosive material. The men are suspected of offering assistance to jihadists entering France after training in the Pakistani-Afghan area.
In a separate operation, eight men and a women arrested in Marseilles are reportedly suspected of involvement in arms and explosives trafficking. Some of those arrested had already served time for "association of criminals in relation with a terrorist organization."