On September 15, FBI Director Robert S. Mueller III and John E. McLaughlin, then acting director of CIA, brought a special note of concern to their daily briefing with President Bush. Fresh intelligence pointed to plans for a terrorist attack before Election Day, bolstering indications that such an assault was possible on US soil, according to accounts of the briefing provided to Mueller’s and McLaughlin’s subordinates.Intelligence officials said there was reason to believe that the plotters may already have arrived in the US. The new data led FBI and other agencies to launch a well-publicised campaign aimed at foiling potential plots, including scanning immigrant neighbourhoods and surveillance of suspected terrorist sympathisers.But five weeks after the effort began, US 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.Intelligence officials stress that they continue to receive reports indicating that Al Qaeda and its allies would like to mount attacks in the US close to the November 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 interviews, immigration arrests and other preventive measures, law enforcement officials say they have been unable to detect signs of an ongoing plot, 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.‘‘You can never say there is not a threat, but we have not found specific evidence of one.’’The situation provides a clear example of the challenges facing the US agencies as they wrestle with foes whose intentions, capabilities and identities remain unclear. ‘‘We remain convinced that Al Qaeda’s allies and sympathisers are intent on striking in the US homeland,’’ said one intelligence official.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 could not tell Bush who or where the suspected plotters were, whether they had evaded screening at US borders. — LAT-WP