On Sept. 15, FBI Director Robert S. Mueller III and John E. McLaughlin, then acting director of the CIA, brought a special note of concern to their daily briefing with President Bush.

Fresh intelligence had arrived pointing to plans for a mass-casualty terrorist attack before Election Day, bolstering previous indications that such an assault was possible on U.S. soil, according to accounts of the briefing provided to Mueller's and McLaughlin's subordinates. What's more, intelligence officials told Bush, there was reason to believe that the plotters may already have arrived in the United States, according to the accounts. The new information led the FBI and other agencies across the government to launch a well-publicized campaign aimed at foiling potential plots before the elections, including hundreds of interviews in immigrant neighborhoods and aggressive surveillance of suspected terrorist sympathizers.

But five weeks after the effort began, U.S. intelligence and law enforcement officials say they have found no direct evidence of an election-related terrorist plot. Authorities also say that a key CIA source who had claimed knowledge of such plans has been discredited, casting doubt on one of the earliest pieces of evidence pointing to a possible attack.

Intelligence officials stress that they continue to receive reports indicating that al Qaeda and its allies would like to mount attacks in the United States close to the Nov. 2 elections, and that such reports have been streaming in since terrorists blew up commuter trains in Madrid days before Spanish elections in March. Yet after hundreds of interviews, scores of immigration arrests and other preventive measures, law enforcement officials say they have been unable to detect signs of an ongoing plot in the United States, nor have they identified specific targets, dates or methods that might be used in one.

"We've not unearthed anything that would add any credence to talk of an election-related attack," said one senior FBI counterterrorism official, who spoke on the condition of anonymity because authorities have been instructed not to talk publicly about the issue before the elections. "You can never say there is not a threat, but we have not found specific evidence of one."

Like so much of the war on terrorism, the possible election threat is distinctly alarming and maddeningly opaque, according to government officials. The situation provides a clear example of the challenges facing the FBI, the Department of Homeland Security and other U.S. agencies as they wrestle with foes whose intentions, capabilities and identities remain unclear.

"We remain convinced that al Qaeda's allies and sympathizers are intent on striking in the U.S. homeland," said one U.S. intelligence official who, like others, spoke on the condition of anonymity because the threat involves classified information. "But the time frame, as it always is, is ambiguous. If we get through the election, it's not like we can walk off the field."

"Until you find the Mohamed Atta of this plot," the official added, referring to the ringleader of the Sept. 11, 2001, hijackings, "how can you stop?"

For their briefing with Bush, McLaughlin and Mueller had only fragments. They were concerned enough that they separated their report on the election dangers from the routine daily synopsis of threat reporting known as the "threat matrix," law enforcement sources said.

Yet the two men could not tell Bush who or where the suspected plotters were, whether they had evaded screening at U.S. borders, which targets they had in their sights, or what weapon they planned to employ. McLaughlin and Mueller could not, in fact, say for sure that the plot existed, the sources said.

A CIA spokesman declined to comment.

Mueller and Attorney General John D. Ashcroft had warned as early as May that al Qaeda may seek to strike close to the elections, but the reports had reached such a pitch in September that officials chose a large-scale response. Their plan called primarily for aggressive, and overt, surveillance of people already under scrutiny for possible terrorist ties. In a few cases, law enforcement officials said, the plan would lead to arrests before the bureau would otherwise have made them. In most others, the FBI and its joint terrorism task forces would do little more than "pull up in traffic and have people staring" at their subjects, as one official put it.

"Even if this guy is not likely to become a suicide bomber, will security benefit by letting the guy know we're watching him?" one official said. The hope is to "dissuade them from doing things they might otherwise have done," the official said.

At the Department of Homeland Security, an immigration unit has detained 120 foreigners so far this month on charges of being in the country illegally, including some who are named in databases of criminal or terrorism suspects, officials said yesterday.

At the FBI, about 2,000 counterterrorism agents have been assigned the task of conducting interviews and following up on leads, with instructions to report to 24-hour call centers in each field office. The disruption plan "is intense up to the election, but they're keeping command posts operational for longer than that," one official said.

The person in charge of the campaign is Patrick Cook, who was summoned to FBI headquarters the day after Bush's briefing, officials said. Reassigned on the spot from his job as a senior official in the Washington field office, Cook moved to the FBI's Strategic Information Operations Center with a mandate to run the national disruption plan.

"They told him his whole job is to prevent an attack before the inauguration," said a sympathetic colleague who works elsewhere. "Which is like being told, 'Make the sky turn purple.' "

The FBI's approach depends on "tripwires" to detect suspicious activity. The system, implemented last year and based on the behavior of the 19 Sept. 11 hijackers, generates alerts if a known subject buys an airline ticket, rents a car or applies for a driver's license -- in his or her own name. The national criminal information database, consulted routinely when local police make a traffic stop, is now capable of sending a "silent hit" to the bureau if the driver is on a watch list.

"If they follow the model of the 19 [hijackers], we'd detect them, I can tell you that," said a high-ranking law enforcement official, who added that he is unable to discuss the screening methods in public.

The FBI and other agencies have also performed exhaustive searches of records on explosives permits, rental storage facilities, crop-dusting airplanes and other specialized areas that have been identified as potential targets of al Qaeda.

Yet law enforcement and intelligence officials frankly acknowledge that their information is limited. "People are so terrified because they can't see clearly anymore," a government counterterrorism analyst said. Because of the success in closing al Qaeda's sanctuary in Afghanistan, the analyst said, "we can't see the training camps, we've driven their communications further underground, and the operators have effectively disappeared."

Even as the government intensified its campaign, authorities discovered that one of the CIA sources they had relied on had fabricated his story, according to several counterterrorism officials. One intelligence official said the revelation "caused us to go back to square one and reassess where the plotting really is."

Other officials, however, played down the source's importance. "It's thought that what he had said was pure misinformation" designed to mislead the government, a different intelligence official said. But, the official added, that did not increase anyone's comfort level, because there are many other sources indicating that al Qaeda wants to launch an attack.

FBI and Justice Department officials said they are still keenly worried about the whereabouts and activities of seven fugitives who were named in May as possible suspects in the planning of an al Qaeda attack. One person of particular concern is Adnan G. el Shukrijumah, a Saudi-born radical raised in Guyana and the United States who has been identified as a valued operative by Khalid Sheik Mohammed, the al Qaeda lieutenant who is in U.S. custody.

Shukrijumah, 29, is a trained pilot who lived in Florida until he fled after the Sept. 11 attacks. He has a $5 million U.S. bounty on his head. U.S. authorities have linked him to numerous possible plots, including an abandoned scheme with U.S.-designated enemy combatant Jose Padilla to blow up U.S. apartment buildings with natural gas. The FBI has fielded numerous reported sightings of him from Morocco to Central America, but none has been confirmed.

"A number of the detainees, when asked 'Can you think of who would be sent to the U.S. for an attack?,' " have named Shukrijumah, a terrorism analyst with the government said. "He's a real threat. He speaks Spanish, English and Arabic; he's totally bought into the plan, and nobody -- but nobody -- knows where he is."

A key component of the disruption plan has focused on scrutinizing immigrants for violations. Among those arrested by Homeland Security in recent weeks was a 28-year-old Saudi who had dropped out of a U.S. university after enrolling last year, according to a news release from the department's Immigration and Customs Enforcement (ICE) bureau. The student, who was not identified, was stopped last year while trying to carry a stun gun onto a U.S. airliner, the release said.

Another former student, a 24-year-old Lebanese citizen, had his visa revoked by the State Department for national security reasons, the release said. He was working in a convenience store and was no longer in school, according to the release. It did not say where the former students were living.

The arrests were made by ICE's Compliance Enforcement Unit, which flagged the suspects with the help of three new systems for tracking visitors: a student database, a system for identifying arriving and departing foreigners, and a program that requires men from two dozen mostly Muslim countries to register.

Not all of the 120 arrests involved security risks. One of those listed, for example, was a South African woman who entered the country this year on a student visa but never enrolled. She was arrested and placed in deportation proceedings but was released with an electronic monitoring bracelet, the news release said.

Staff writers John Mintz and Mary Beth Sheridan contributed to this report.

New security precautions include the closing of the 15th Street sidewalk beside the Treasury Building, forcing pedestrians to walk on a traffic lane.FBI Director Robert S. Mueller III, left, and then-acting CIA chief John E. McLaughlin briefed the president last month about a possible attack.