"There will be another terrorist attack. We will not be able to stop it," FBI Director Robert Mueller told a group of local prosecutors meeting in suburban Alexandria, Va. "It's something we all live with."
Mueller's sobering predictions — "I wish I could be more optimistic" — came one day after U.S. Vice President Dick Cheney said it was almost a certainty the United States would be attacked again by terrorists.
And while shopping malls, apartment complexes and public buildings have all been mentioned as potential targets, one U.S. official told CBS News the administration's greatest fears still center around commercial aviation, specifically the possibility a suicide bomber would check a suitcase with a bomb aboard a domestic flight.
CBS News Correspondent Jim Stewart reports airlines are rushing to obey a congressional order that they inspect all baggage - both hand-carried and checked - but are not expected to fully comply until the end of the year.
Officials described the latest threat warning as a product of "communications intercepts" including "Internet chatter," "hard intelligence retrieved from Afghanistan" and the "results of prisoner interrogations."
The latest intelligence shows a marked increase in activity by Osama bin Laden's al Qaeda network over the past few weeks, suggesting new attacks may again be in the offing, said a senior U.S. official, speaking on condition of anonymity.
Defense Department spokeswoman Victoria Clarke stressed that despite major successes against al Qaeda in Afghanistan, where a U.S.-led military campaign has destroyed the group's main camps, it and other groups remained a threat.
"We have always said this is about more than one person, one network and certainly is about more than Afghanistan," Clarke told a news briefing.
Islamic groups like Hezbollah and Egypt's Islamic Jihad could be planning to attack the United States and may be more able to do so than al Qaeda, the Senate Intelligence Committee chairman said Monday.
"Our enemy is not al Qaeda alone," Sen. Bob Graham told NBC. "There are several international terrorist groups which have abilities, in some cases greater abilities than al Qaeda and a similar desire to attack the United States."
"Groups like Hezbollah, the Egyptian Islamic Jihad" were two organizations with the capability and desire to attack the United States, said the Florida Democrat.
Neither group has been linked directly to activities on U.S. soil, and such an attack would represent a major change in their tactics.
Graham said in a separate CNN interview that he was "almost certain" another attack would take place within three to five years.
He said he was especially concerned about security at the nation's seaports and mentioned a recent instance of 25 "extremists" who had entered the United States.
"(They) jumped on ships outside the United States, hid in the container cargoes until they got to the United States and then disembarked and they've been lost in the American population," he said.
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.
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.
Under fire for its handling of terrorism intelligence before the Sept. 11 attacks, the White House in recent days has increased the volume of warnings.
The Bush administration is fighting Democratic-led efforts to have an independent commission rather than existing congressional intelligence committees study its performance before Sept. 11.
Calling for a commission, House Minority Leader Richard Gephardt, a Missouri Democrat, said various branches of government possessed information last summer about the Sept. 11 terrorists.
"Unfortunately, these and perhaps other documents were not compiled in a manner that enabled our counter-terrorism experts to use them most effectively," he said.
CBS News Chief White House Correspondent John Roberts reports the White House has pledged to co-operate with a joint investigation of the House and Senate intelligence committees.
The committees, however, want more co-operation by the FBI and CIA on sensitive information. Committee members say the inquiry will be "worthless" unless the agencies are more forthcoming. Committee leaders are expected to meet Tuesday with Attorney General John Ashcroft and later with CIA director George Tenet to discuss greater sharing of intelligence.