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.
"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.
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.