Alexandria, Va. ? Walk-in suicide bombers like those who have attacked public places in Israel will hit the United States eventually, 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 meeting in suburban Alexandria, Va.

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.

Mueller’s prediction follows Vice President Dick Cheney’s warning Sunday that because no specific information is available, the United States is finding it difficult to respond to the latest intelligence hints that al-Qaida may be planning another attack.

Mueller 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.