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.

New security precautions include the closing of the 15th Street sidewalk beside the Treasury Building, forcing pedestrians to walk on a traffic lane.
(Jonathan Ernst For The Washington Post)
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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.