Larry Mefford, assistant director of the FBI counterterrorism division, said after appearing before a Senate Judiciary Committee panel on terrorism that the investigations run "the whole gamut" from people identified as al-Qaida operatives to uncorroborated tips from citizens.
"We know this: The al-Qaida terrorist network remains the most serious threat to U.S. interests both at home and overseas," Mefford told the panel. "That network includes groups committed to the international jihad movement and it has demonstrated the ability to survive setbacks."
In the United States, the FBI worries most that it has not definitively located members of al-Qaida cells who might be able to pull off an attack like the 19 hijackers who flew planes into buildings on Sept. 11, 2001, he said.
Asked if he believed such a cell could be poised for a new attack, Mefford said, "Not that I'm aware of."
The FBI has had greater success in finding people suspected of providing logistical or recruiting help to al-Qaida. Mefford cited as an example last week's guilty plea by Ohio trucker Iyman Faris, who admitted to conducting surveillance for a possible attack on the Brooklyn Bridge and plotting to derail trains.
Earlier this week, President Bush designated as an enemy combatant Ali Saleh Kahlah al-Marri, a Qatari man living in Illinois who the government claims provided financial and other support to new al-Qaida arrivals in the United States.
"I know the logistical cells are here," Mefford told reporters. "The question is whether the most dangerous cells are here."
One difficulty is that many al-Qaida members use multiple aliases and "jihad" names that are difficult to track. The FBI earlier this year found the true identity of one suspected cell member, Mefford said: Adnan G. El Shukrijumah, a Saudi-born man with Florida ties who is the subject of a global manhunt.
"We don't know where he is," Mefford said.
Mefford told the Senate panel that arrests of senior al-Qaida leaders and disruption of its sanctuary in Afghanistan have made it more difficult for the organization to mount major attacks. The FBI is concerned that could lead to more random, smaller-scale attacks against lightly secured targets.
He also said that al-Qaida continues efforts to recruit U.S. citizens and non-Arab operatives who could more easily escape detection and slip through new security measures at U.S. borders.
"They understand the benefits of having this type of asset, somebody who can travel under the radar screen," Mefford said.
Sen. Charles Schumer, D-N.Y., raised several questions about whether Muslim clerics are spreading a version of extreme fundamentalist Islamic teachings in American prisons and among Muslims in the U.S. military.
Schumer said this interpretation of Islam, known as Wahabbism, has spread from Saudi Arabia and preaches "hate, violence and intolerance" toward moderate Muslims, Jews and Christians. Most al-Qaida members subscribe to these teachings, he said.
"My fear is, if we don't wake up and take action now, those influenced by Wahabbism's extremist ideology will harm us in as of yet unimaginable ways," Schumer said.
Mefford said that the FBI and Bureau of Prisons, along with state corrections officials, are actively working on ways to prevent al-Qaida from recruiting in prisons through proselytizing by Islamic clerics tied to terrorism or extremist teachings.
"That's an ongoing project," he said.