Al Qaeda (searchoperatives in custody are providing only the barest information on the terror network's activities, often proving too canny to be broken down quickly during interrogation, Homeland Security Secretary Tom Ridge (searchsaid Wednesday.

Still, through the prisoners and other intelligence sources, U.S. understanding of the network is growing, Ridge said in an interview with The Associated Press.

"We are still their No. 1 target," Ridge said. "If there's a consistent theme to all the intelligence we've received over the past two-plus years, it's their interest in undermining the United States economy, with an emphasis on aviation and infrastructure."

Ridge provided a glimpse into the ongoing interrogation of top Al Qaeda operatives such as Khalid Shaikh Mohammed (search), Abu Zubaydah (searchand others captured in the worldwide dragnet since Sept. 11, 2001.

"They're schooled. They're practiced. And extracting information from them is a very time-consuming, arduous, difficult, painstaking, complex task," he said.

The interrogations are being conducted by the CIA and foreign security services, with assistance from the FBI and the U.S. military. The results are provided to Homeland Security.

Ridge didn't discuss interrogation methods, or the locations of the top prisoners, but suggested they have not provided the wealth of information counterterrorism officials had hoped for.

"The higher they are in the chain of command, the tougher the show is," Ridge said. "We extract bits and pieces from them. It is a very complex, methodical process. One little bit of information we may get from one of the big fish or from a minnow can lead to another little bit of information."

A few prisoners have provided gratifying reports.

"We've been told by a couple of detainees that a couple of potential operations were put on the back shelf because of enhanced security," Ridge said.

But the Al Qaeda operatives still at large are also learning and adapting, making them harder to track.

"They watch and read and know what's going on the United States," he said. "They've changed in many respects their communications patterns -- how they communicate, how they talk to each other."

At this point, officials don't have significant evidence that Al Qaeda might try a strike on the second anniversary of the Sept. 11 attacks. The color-coded terror threat system is staying at yellow, the middle level on the five-color scale, Ridge said. Yellow signifies an elevated threat.

Last year, operatives plotted strikes against U.S. embassies in southeast Asia, but these attacks didn't go forward, counterterrorism officials say.

Ridge said he wants to employ a much more targeted set of alerts -- to specific regions or sectors of the economy, for example -- but the intelligence has not provided enough clarity for Homeland Security to take that step.

Six months ago, Ridge's fledgling department took over federal disaster response, immigration, customs, and security at U.S. borders, ports and airports. Ridge, a former infantry sergeant and Pennsylvania governor, is managing the largest reorganization of the federal government in 50 years.

His department is also receiving and analyzing intelligence gathered by the CIA and other agencies, looking for places in the United States that terrorists could strike.

"We are very confident in the administration that we have been able to thwart, foil or delay at least some terrorist acts within the United States," Ridge said.

He noted Al Qaeda has suffered a number of reversals since Sept. 11, although it has been linked to deadly bombings in southeast Asia, the Arabian peninsula and North Africa.

"We've decapitated quite a bit of their operational side," he said. "The freezing of substantial assets makes it more difficult for them to operate."

On other matters:

-- Ridge gave his department a passing grade for its performance during the blackout that paralyzed much of the northeastern United States and Canada. The true test will come when his department must deal with simultaneous disasters, either natural or terrorist-caused, he said.

-- On airport security, Ridge said the next big challenge for the Transportation Security Administration will be to install van-sized electronic bomb-detection machines as part of airports' baggage-handling systems.

The agency put some machines in airport lobbies in order to meet a congressional deadline to screen all checked bags by the end of last year, Ridge said.

"We're going to have to go back and frankly retrofit some of those airports," he said. "It's not the most efficient way to do things. There was a mandate to get it done."

-- Officials are still wrestling with how to distribute counterterrorism money to the states, he said. At issue is how much to weigh factors like population density, the presence of landmarks and threat intelligence in deciding who gets more federal dollars.