U.S. Plans Law Enforcement 'Surge' On Trains

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.

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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.

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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.