Some 50,000 people travel from Europe to the United States each day under a visa waiver system that has come under increasing scrutiny as terror concerns rise.
Most travelers from Western Europe don't need visas to come here, which means their backgrounds are subject to minimal inspection. The waiver program was designed to foster the free flow commerce and tourism, but it's now increasingly viewed as a major vulnerability in the war against terrorism.
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"We've seen over the last couple of years activity in Europe which is disturbing," Homeland Security Secretary Michael Chertoff told ABC News in an interview last week.
"I've spoken publicly about my concern that Europe could be a platform against the U.S.," he said.
There's overwhelming evidence, U.S. officials say, that Europe could be the gateway for the next terror attack in the United States.
Incidents include the "shoe bomber," British citizen Richard Reid, attempting to blow a flight from Paris to Miami out of the sky in 2001; Madrid's train bombings in 2004 that killed nearly 200 and wounded nearly 2,000; a failed plot in Germany last year targeting trains; and the bombings and attempted bombings in London in each of the last three years.
A new intelligence report -- which is still classified -- flatly states that al Qaeda wants to use Europe as a staging ground to send terrorists to the United States.
The National Intelligence Estimate's key findings were made public earlier this month, and one of the conclusions states: "Although we have discovered only a handful of individuals in the United States with ties to al-Qa'ida senior leadership since 9/11, we judge that al-Qa'ida will intensify its efforts to put operatives here."
"We are very concerned about the capability of terrorist groups to use Europe as a venue and a launching point for terrorists -- bringing terrorists into the United States," CIA director of intelligence John Kringen told Congress at a July 11 House Armed Services Committee.
"We are very concerned about the connection that clearly exists between British citizens in some cases -- British immigrants in other cases coming out of Pakistan," he continued. "And so that connection between Pakistan, the U.K. and then the potential for those individuals to get into the United States is a matter of exceedingly high concern to our agency."
ABC News has reported on the recent surge of terror training camps in the tribal regions in Pakistan, a growing concern for U.S. officials.
There is also a growing fear of the proliferation of homegrown extremists in European countries themselves.
"We're increasingly looking at Europe as the threat of homegrown European radicals increases and also the danger that they are getting trained in either person or over the Internet," Chertoff said.
New Measures Proposed
The Department of Homeland Security is proposing new measures to tighten the security on European passengers, including a requirement that Europeans register online two to three days before traveling to the United States.
Other elements of the plan include pressing countries allowed to use the waiver to promptly report missing and stolen passports, and urging the Europeans to develop specially marked travel documents, which would be less susceptible to fraud.
The U.S. government also wants to gather information on the traveler's method of payment, their telephone number, seat number, and whether they have a special meal request.
The goal is to give U.S. officials more time to conduct background and intelligence analysis -- in effect, more time to hunt for ties to terrorism. Currently, the United States doesn't get this information until the travelers are on the airplane, which has at times led to terror scares and midflight diversions.
"There have been many examples over the course of the last three years where individuals have already been in transit to the United States when we have discovered that they're the type of individual that, for a specific reason, we didn't want in our country," said former senior DHS official George Foresman.
European critics point out the U.S. government will keep the information for decades -- 40 years -- and say the proposed security changes smack of Big Brother.
"The Chertoff vision is of data-mining and profiling on the basis of past and assumed future behavior and stereotypes of potential terrorists," European Parliament Member Sarah Ludford said in a debate on the issue last fall.
"This takes us well beyond the simple checking of people against watch lists," she continued.
Ludford also called for an explanation from the United States of how the collected data would be used and of the techniques used to collect it. "We need strict and legally binding purpose- and access-limitation provisions," she said.
But some officials tell ABC News they're frustrated that the proposed measures -- which they see as common sense moves -- are not already in place. And Chertoff said he sees it as a necessary step to prevent potential terrorists from entering the country.
"We've been pushing very hard and successfully to get more and more information about travelers coming from Europe so that doesn't become a launching point for another attack against this country," he said.
Officials say the idea is not to close America's doors, but to have a better understanding of who's coming in well before they depart.
"This is not an anti-American thing," Flight International's operation and safety editor David Learmount said of European skepticism of the proposal. "It's just a sense of resignation that the Americans are wasting an awful lot of time and money and they are turning America into a kind of fortress. They [Americans] are pulling up the drawbridge when they should be building bridges."