Washington ? Adding to the Bush administration’s stark warnings about terrorist attacks, FBI Director Robert S. Mueller III said Monday that a walk-in suicide bombing like the ones that have terrorized Israelis is “inevitable” in the United States.

Mueller, in a closed session at the meeting of the National Association of District Attorneys in Alexandria, Va., acknowledged that the FBI and other intelligence agencies have difficulty infiltrating terrorist groups built on fanaticism.

“There will be another terrorist attack,” Mueller said, according to an Associated Press account. “We will not be able to stop it. It’s something we all live with.”

Mueller’s assessment came a day after Vice President Dick Cheney warned of “a real possibility” that Palestinian-style suicide bombers would carry out attacks within the United States.

One Justice Department official said Monday that Mueller’s statements about suicide bombers are “consistent with the patterns we’ve seen overseas, and consistent with the information we’ve been seeing over the past few weeks.”

Bush administration officials have said in recent days that intelligence analysts have noted a surge in communications among al-Qaida terrorist operatives regarding an assault. Officials said the activity is the greatest since the Sept. 11 attacks, which the United States has blamed on al-Qaida and its leader, Osama bin Laden.

However, Cheney and national security adviser Condoleezza Rice said the intelligence provided no specific information about an attack or whether one is imminent.

After recent bombings in Russia linked to al-Qaida-allied Chechen militants, the FBI issued a caution to U.S. terrorist task forces last week that al-Qaida terrorists might rent apartments and bomb them.

Despite such warnings, the Bush administration has not moved to put the nation on a higher stage of alert. Homeland security officials unveiled a five-tiered, color-coded alert system in March and since have kept the nation at stage “yellow,” connoting a significant risk of attack. The next level up, orange, would mean officials believe there is a high risk of a terrorist strike.

Democrats push probe

The warnings follow fierce debate on Capitol Hill over how aggressively Congress should be allowed to probe possible intelligence failures before Sept. 11. Democratic leaders said Monday that they plan to oppose the White House’s effort to confine a congressional inquiry to the joint investigation already under way by the House and Senate intelligence committees, which have access to classified information.

House Minority Leader Richard A. Gephardt, D-Mo., said he will sponsor a bill to establish an independent commission to analyze the Sept. 11 attacks. “It’s imperative that in addition to improving our ability to gather intelligence, we also do a better job of coordinating, disseminating and acting upon this intelligence,” Gephardt said.

Senate Majority Leader Thomas A. Daschle, D-S.D., might begin moving a similar bill as soon as this week, an official said.

Cheney and other administration officials have refused to release publicly a copy of a CIA briefing paper presented to President Bush Aug. 6, which warned that al-Qaida terrorists might hijack U.S. airplanes.

Mueller and the Justice Department have rebuffed congressional requests for a copy of a July 10 memo from a Phoenix agent to FBI headquarters. It warned that bin Laden might be using U.S. flight schools to train terrorists.

Agencies pointing fingers

Sen. Patrick J. Leahy, D-Vt., chairman of the Senate Judiciary Committee, is considering whether to issue Mueller a subpoena for the document this week, sources said.

The FBI did not share the Phoenix memo with other agencies or with FBI agents in Minnesota, who were scrambling to determine why alleged Sept. 11 conspirator Zacarias Moussaoui had been taking flight lessons before his arrest last August.

FBI officials have said that the information on Moussaoui was shared with several agencies, including the CIA and Federal Aviation Administration. But the FAA, which had issued a flurry of hijacking warnings last summer, did not alert airlines to the case.

FAA spokesman Scott Brenner said Monday that the FBI “said they didn’t think he was involved with anybody else, that there was not an imminent threat, and, most importantly, he was in jail.”