A formal US travel alert warning Americans against possible terrorist attacks was based on a "growing body of information" collected over time, a US official said Monday.
American and European authorities were searching for suspects after the United States on Sunday issued an unusual travel alert warning of "the potential for terrorist attacks," with European railways, subways and tourist spots possibly under threat.
"A growing body of information on terrorist plotting -- gathered over time -- factored into the decision to issue the alert," a US official told AFP.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
The precise timing and targets remained unclear, said the official, who spoke on condition of anonymity.
"American and European officials are working hard -- and closely together -- to disrupt any attacks that might be planned," the official added.
European security officials last week said a plot or plots had been uncovered that suggested 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, a former CIA officer said.
"My assumption is that the European governments and the US government have something more than just supposition here," said Bruce Riedel, a veteran of the CIA who has advised the US administration on Afghanistan and Pakistan.
Such a travel advisory "only is issued when there is some reason that goes beyond theory, to alert the public," he said.
German media, meanwhile, reported that US and European intelligence agencies are closely following the questioning of an Afghan-German suspect detained by US forces in Afghanistan, who may have provided information about plots in Europe.
Ahmed Sidiqui, of Hamburg, reportedly was part of the Islamic Movement of Uzbekistan and told American interrogators that some operatives in the plot may already be in Europe.
Although the US spy agencies took the suspect's account seriously, German officials had adopted a more skeptical view, according to Der Spiegel magazine.
The suspect reportedly has links to at least four members of Al-Qaeda's Hamburg cell, which played an instrumental role in the September 11, 2001 attacks.
Officials from Germany's intelligence agencies are due to travel to Afghanistan to interrogate the suspect and a German diplomat had met with him on Sunday, the magazine reported.
