Chertoff's comments on Capitol Hill comes as the country is entering a potentially vulnerable period with the presidential nominating conventions coming up next month; the presidential election in November; and the transition to a new administration in January - all of which may be attractive targets for terrorists.
In his last scheduled appearance before the House Homeland Security Committee, Chertoff said that the more time and space al Qaeda and its allies have to recruit, train, experiment and plan, the more problems the U.S. and Europe will face down the road.
"The terrorists are deliberately focusing on people who have legitimate Western European passports, who don't appear to have records as terrorists," Chertoff told lawmakers. "I have a good degree of confidence we can catch people coming in. But I have to tell you ... there's no guarantee. And they are working very hard to slip by us."
Chertoff and other intelligence officials have delivered similar warnings before, and he offered no new information about specific threats or an imminent attack.
Chertoff reiterated his concern that terrorists could sneak radiological material into the country on small boats or private aircraft. This material could be used to create an explosive device known as a "dirty bomb."
The Homeland Security Department has a strategy to protect against this small boat vulnerability and is testing radiation detection equipment in Seattle and San Diego ports.
Chertoff said that getting out a regulation to prescreen and enhance security of general aviation aircraft coming to the U.S. from overseas is one of his top priorities.
He also said he expects to approve new radiation detection technology this fall.
Responding to a question from Rep. Al Green, D-Texas, Chertoff dismissed any rumor that he is on a list of potential running mates for Sen. John McCain, the presumptive Republican presidential nominee. Chertoff quipped that the only list he has for next year is a list of vacations.
Chertoff's term as the country's second Homeland Security Secretary ends when a new administration takes over the White House in January.