A senior FBI official, speaking on condition of anonymity, said some of the leads were culled from interviews with thousands of individuals that agents have conducted in the Muslim community.
The official would not be more specific, but said the FBI continues to have misgivings about possible al-Qaida intentions to launch an attack with the goal of affecting the elections.
Several people have been taken into custody recently on charges not related to terrorism, but officials are investigating whether they may have been involved in terror activities, said another law enforcement official, speaking on condition of anonymity.
As for the person who warned the CIA, at least some of that individual's reporting no longer is seen as credible, said a U.S. intelligence official, speaking on condition of anonymity. The official stressed, however, that a number of other sources point to terrorist activity around the election season.
The official also said no concrete plot has been discovered so far with specific mention of time, place, method or identity of would-be attackers.
James Carafano, a homeland security expert with the conservative think tank Heritage Foundation, said there's little doubt that al-Qaida wants to send a message with an attack.
"I think they're probably less concerned with affecting the outcome of the election, whether Bush or Kerry wins, but it would be more prestigious, the capability to do something before the election and really undermine the confidence of the United States," he said.
Top government officials have warned since the spring about al-Qaida's desire to attack during the election with the hope of orchestrating something similar to the March commuter train bombings in Madrid. The bombings, which killed more than 190 people, were a factor in the ouster of Spain's former ruling party.
The FBI has partnered with American Muslims and others as part of what the agency describes as a prevention plan aimed at disrupting an attack. Thousands of agents from bureaus across the country are participating in the effort.
FBI agents have interviewed thousands of people considered possible sources of information as well as about 10,000 operators of self-storage facilities, which are considered likely places to hide bomb-making materials or other items that could be used in an attack.