Americans should brace themselves for "inevitable" walk-in suicide bombings like those that have occurred in Israel, FBI Director Robert Mueller said Monday.
"I think we will see that in the future, I think it's inevitable," Mueller said in response to a question during a speech to the National Association of District Attorneys.
"There will be another terrorist attack," Mueller told the group in his opening remarks. "We will not be able to stop it. It's something we all live with."
Mueller made his remarks one day after Vice President Dick Cheney urged Americans to stay vigilant because the chance of more Al Qaeda terror attacks against U.S. targets is "almost a certainty."
"It could happen tomorrow, it could happen next week, it could happen next year, but they will keep trying. And we have to be prepared," Cheney said on Fox News Sunday.
The Bush administration's predictions of certain terror attacks are based in part on new intelligence that shows an increase in communications and other activity over the past few weeks by Usama bin Laden's Al Qaeda network
The activity suggests that new attacks may be in the works, said a senior U.S. official, speaking on condition of anonymity.
The official portrayed the intelligence as a new peak in a high-and-low cycle of terrorist threats that counterterrorism authorities have tracked for years. The last peak was in March, when Al Qaeda financial activity and communications stepped up. That was linked to Al Qaeda leader Abu Zubaydah, who was subsequently captured in Pakistan.
Another peak in threat reporting took place last summer and is now regarded as evidence of Al Qaeda's preparations for the Sept. 11 attacks on New York City and Washington. Other peaks have come and gone, and no attack has taken place.
Mueller said the degree of fanaticism an informant must exhibit to get into the inner circle of a terrorist group makes it difficult for law enforcement agencies to penetrate such organizations and prevent such attacks.
"I wish I could be more optimistic," he said.
He said law enforcement has been somewhat successful in combating acts of terrorism in Northern Ireland by developing sources who could provide information about terrorist plans and by using electronic surveillance.
But he said the difficulty of getting informants inside terrorist groups targeting the United States makes it much harder to obtain advance information.
Cheney said Sunday he sees "a real possibility" that walk-in suicide bombers may hit the United States if those who have attacked Israel succeed in changing the situation in the Middle East.
"Terrorism is an evil, pernicious thing, and it is one of the biggest challenges we've ever faced as a nation," Cheney said.
At the prosecutors meeting, Mueller said the FBI now believes that "an Al Qaeda bomb maker" constructed the shoe bomb that Richard Reid had when he was apprehended aboard a flight from Paris to the United States in December.
Mueller made the comment in describing how the FBI is increasing its recruitment of scientific experts to help in terrorism investigations and is "centralizing analytical capability" to coordinate evidence gathering.
Mueller also said the arrest of Abu Zubaydah during raids in Pakistan in March was the result of a joint FBI-CIA operation. He cited the raids as an example of how the traditional wall between the FBI and the CIA is coming down as the two agencies battle terrorism.
The Associated Press contributed to this report.