Michael Chertoff said in an interview with British Broadcasting Corp. radio that his fears were based on how easy it is for Europeans travel to the United Sates. He said there was also evidence of homegrown terrorism in European countries.
"One of the things we've become concerned about lately is the possibility of Europe becoming a platform for a threat against the United States," Chertoff said.
Washington has no plans to start asking Europeans to apply for visas to travel to the United States but they could be required to register online before traveling, Chertoff said.
"We have the visa waiver program, which allows most Europeans who come to be tourists to come without visas - that means the first time we encounter them is when they arrive in the United States, and that creates a very small window of opportunity to check them out," he said.
The U.S. concerns are based on cases of homegrown terrorism in Europe - such as the March 2004 bombings in Madrid that killed 191 people, the deadly 2005 transport bombings in London and a failed attempt to blow up two commuter trains in Germany in 2006.
"We have watched the rise of homegrown terrorism," Chertoff said. "We are obviously mindful of the Madrid bombings, the attempted bombings in Germany, and that suggests to us that the terrorists are increasingly looking to Europe as both a target and a platform for terrorist attacks."
He said there was a "certain sense of complacency" about terrorism in the United States, suggesting the Bush administration had been a victim of its own success in thwarting any attacks since the Sept. 11 attacks.
"But when I lift my eyes and I look around the world, and I look at what happens in Britain, in Germany, in Spain, in Bali, in Pakistan, I don't see terrorism going away; I see an al Qaeda that's evolving," he said.
Still, as the fifth anniversary of the creation of his Homeland Security department approaches, Chertoff told the BBC, "Our ability to know who is coming across our borders, and to check them and to take their fingerprints now, is far better than it was five or six years ago. We have deployed almost twice as many border patrols as we had five or six years ago. We've put up literally hundreds of miles of fencing and barriers. All of these are very concrete steps forward that have made our security much better than it was."
© MVIII, CBS Interactive Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report