Despite the intelligence harvest, however, U.S. officials are at a loss to understand why al-Qaida failed to mount more attacks after Sept. 11. Even if major attacks were impossible, they wonder why they didn't strike "soft targets" like subways or shopping malls, in major cities.

"We thought the first thing they would do is pull a Japan in the subway, because it works," the intelligence chief said, alluding to the 1995 sarin gas attack on the Tokyo subway by the Aum Shin Rykyu religious sect.

"We're very vulnerable to it and it could easily happen," he added.

The government's success in cracking down on al-Qaida has helped flush out information in many different ways, but it has also created some intelligence problems. Before Sept. 11, bin Laden's terrorist cells shared information on a strict, need-to-know basis, which allowed the attacks to take U.S. intelligence agencies completely by surprise. Since then, however, the al-Qaida organization, battered by thousands of arrests and the crippling of its banking channels here and abroad, has resorted to cellphones and e-mail to communicate. The ironic result has been to overwhelm U.S. intelligence with chatter that makes it hard to winnow real threats from cheerleading by al-Qaida sympathizers or wannabes, sources said.

Stymied, the FBI and Justice Department have pursued a strategy of picking up anyone remotely suspected of terrorist activities and sorting out the guilty from the innocent later.

"It's mostly the Bureau, together with local cops. But the Bureau's got 3,500, 4,000 agents just working this," the intelligence official said. "They're all over the place," arresting suspects "left and right, all over the place. Every day I get new list."

The suspects have been almost uniformly Arab, he said.

"Pretty much, except maybe for a very, very, very extremely few cases. You got a lot of collateral in there, too. You got copycats, you got these guys that, you know, have vendettas or whatever, against the U.S. government, and they got mixed in."

An FBI agent in the northeastern U.S. agreed.

"At the height of this thing, God help you if you stood near Niagara Falls with a camera, you know, and even looked Middle Eastern. We made a lot of stops on people, just to find out what the hell they were doing."

In this agent's jurisdiction, however, "A hundred percent of it turned out to be crap. I can't speak for all the field offices, but as far as I know, we haven't come up with any smoking gun, a guy standing near Hoover Dam with the pin pulled."

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