The United States is preparing to beef up law enforcement presence on trains and transit systems amid US travel alerts for a possible terrorist attack in Europe.
A US official told ABC News however that the exercise was "long-planned" and "not connected in any way" to the alert for Europe.
The formal US travel alert warned Americans about possible terrorist attacks in Europe as American and European authorities were searching for suspects, with European railways, subways and tourist spots possibly under threat.
European security officials last week said a plot or plots were uncovered suggesting Al-Qaeda militants were planning to stage simultaneous strikes in Britain, France and Germany, with a possible Mumbai-style attack in the works.
The US travel alert, which was promptly backed up by British and French officials, was unusual and suggested concrete intelligence had emerged, according to a former CIA officer.
Such a travel advisory "only is issued when there is some reason that goes beyond theory, to alert the public," said Bruce `Riedel, a veteran of the CIA who has advised the US administration on Afghanistan and Pakistan.