By Susan Schmidt and Bob Woodward

US intelligence officials have told members of Congress there is a high probability that terrorists associated with Osama bin Laden will try to launch another major attack on American targets here or abroad in the near future.

Based on what officials described as credible new information, the FBI and the CIA have assessed the chances of a second attempt to attack the United States as very high, sources said yesterday.

At a briefing October 2, in response to a senator's question about the gravity of the threat, one intelligence official said there is a "100 percent" chance of an attack should the United States strike Afghanistan, according to sources familiar with the briefing.

One senior official said some of the new intelligence is "very real." But the official cautioned that some of it may be braggadocio or even disinformation designed to discourage the United States from retaliating for the Sept. 11 attacks on New York and Washington.

The new information is worrisome enough that officials at the White House, the Justice Department and the State Department have huddled in recent days to figure out the best way to communicate their concern to the public, a source with knowledge of those discussions said.

The concern about another attack is based on intelligence from sources in England, Germany, Afghanistan and Pakistan, according to a source familiar with what congressional intelligence committees have been told. Egyptian, Somali and Pakistani elements of bin Laden's network are thought to be involved.

Members of the intelligence committees declined to comment on the briefings they have received, which are classified. But their public comments, and remarks by Attorney General John D. Ashcroft on Sunday, highlight the danger the country continues to face.

"We have to believe there will be another attempt by a terrorist group to hit us again," Sen. Richard C. Shelby (Ala.), ranking Republican on the Senate intelligence committee, said yesterday. "You can just about bet on it. That's just something you have to believe will happen."

Shelby declined to discuss specific intelligence information on the plans of bin Laden's Al Qaeda terrorist network that were provided in a classified briefing Tuesday by counterterrorism officials from the FBI, CIA and the Defense Intelligence Agency.

Ashcroft warned earlier this week that there is a "likelihood of additional terrorist activity," and that the "risks go up" once the United States responds with military action. "We think that there is a very serious threat of additional problems now," Ashcroft said. "And frankly, as the United States responds, that threat may escalate."

The Justice Department sought to play down that warning slightly Monday, after Ashcroft's words received more media attention than officials had expected.

"Ashcroft's and [Secretary of State Colin L.] Powell's people and the White House are working on how to word their warnings," a source familiar with multiagency discussions said. "The government doesn't want to panic people." But, he added, "The government is definitely preparing for a counterstrike by bin Laden."

Officials at the White House declined to comment yesterday.

Government officials are fearful of attacks at any of hundreds or thousands of locations, including symbols of American power and culture, such as government buildings in Washington and centers of entertainment. They are concerned about truck bomb and car bomb explosions that could be detonated near natural gas lines, power plants and other sites that one source described as "exposed infrastructure."

The FBI has taken a particular interest in crop-dusting airplanes for fear they could be used in a chemical or biological weapons attack.

Mohamed Atta, one of the suspected leaders of the Sept. 11 attack, expressed a keen interest in the planes. Zacarias Moussaoui, a French-Moroccan man in custody as a material witness, reportedly had materials about crop dusting in his possession when he was detained in August.

The overriding goal, a senior official said, is to make the United States a "hard target" for terrorists.

But US intelligence and law enforcement agencies do not have specific information on the nature of future attacks.

The Coast Guard is boarding and searching ships in New York, Boston and other harbors, and security has been stepped up around nuclear power plants, oil pipelines, refineries and other potential targets.

The FBI has found no links between any of the 19 alleged hijackers or their possible accomplices and any of the 1,000 to 2,000 suspected terrorist sympathizers in this country, including known Al Qaeda supporters, lawmakers were told.

The group that conducted the Sept. 11 attacks and anyone who might have helped it operated as a closed unit and there may be other such cells as yet undetected by law enforcement, some members of Congress were told.

"The investigative case has to take a back seat to preventing the next terrorist act," a senior law enforcement official said. "That comes right from the top, from the president of the United States on down."

In preparation, the FBI has a plan in place to go "full tilt" for 72 hours whenever the president decides to make a move against bin Laden, al Qaeda or Afghanistan's ruling Taliban government, the official said. At the investigation's command center in FBI headquarters, a team of analysts and agents has been working around the clock sifting through reports of potential threats since Sept. 11.

US officials acknowledge it is difficult to understand the motivation behind some of the threats they have learned about.

In response to threats from bin Laden's network that were detected in June and July, for example, officials made decisions to abandon some US embassies and to move Navy ships in foreign ports out to sea. Now, officials have concluded, the threats may have been disinformation designed to occupy officials' attention, or to allow bin Laden operatives to observe American counterterror lockdown methods, a knowledgeable source said.

Shelby said law enforcement agencies believe terrorists will do something unexpected, and thus the agencies are trying to think "out of the box" in anticipating what might be ahead. However, he noted, bin Laden has been known to return to the same targets repeatedly, such as the World Trade Center, which terrorists with possible ties to bin Laden's group bombed in 1993.

In 1999, a terrorist cell linked to bin Laden was thwarted in what one participant later testified was a plot to bomb Los Angeles International Airport.

A senior government official said yesterday that if al Qaeda follows its normal pattern, "other attacks are in various stages of planning." The US embassies in Kenya and Tanzania, which were bombed in 1998, were first surveiled as targets in 1994, according to court testimony earlier this year.

The government official, who spoke on the condition of anonymity, said bin Laden's terrorist organization "likes to mix tactics and targets."

Under that theory, more airplane hijackings seem less likely, because security has been increased. Ground-based operations, he said, seem more probable.

Washington Post October 5, 2001; Page A01