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WASHINGTON -- Despite deep skepticism about the credibility of captured Al Qaeda leader Abu Zubaydah, the U.S. intelligence community is taking seriously his claim that his organization has the capability of building a radioactive "dirty bomb," a U.S. official said Tuesday.

American intelligence agents have undertaken a widespread search for evidence to corroborate the statement made to U.S. interrogators in an undisclosed location where the Pakistani militant has been held since his arrest last month, the official said.

"The United States remains a nation that is at war," said White House spokesman Ari Fleischer at his daily briefing Tuesday. "On the one hand, we have been very fortunate that there has been a real lull, that there have been no incidents taking places in the United States. ... But no one should be under any illusions. We have an enemy that is trying to hit us and strike us."

Zubaydah, who is believed to have been Osama bin Laden's chief of operations, said Al Qaeda had given high priority to making such a bomb and using it against important targets in the U.S.

"The question of whether or not they have acquired a dirty bomb ... is something that we have previously identified as something we know they wanted to do," Fleischer said.

Although Zubaydah may be practicing psychological warfare, the threat has to be taken seriously because of the ease with which such a bomb can be constructed and detonated, the U.S. government official said.

"He has a reputation for being much less than truthful," this official said. "You can launch a terrorist attack just by claiming something and causing panic. This kind of disinformation may be what he's trying to do.

"But we're taking this very seriously. It doesn't take much skill to make one of those things."

Unlike the far more complicated and dangerous atomic bomb, which derives its enormous destructive power from initiating an explosive nuclear chain reaction, a dirty bomb can be made by simply combining radioactive material with explosives.

"The idea is to spread radioactive elements over a wide area," said Peter Stockton, former special assistant to the energy secretary for nuclear security. "You can use radioactive waste from medical labs. You simply fill the material with C4 explosive and set it off in a crowded place."

Stockton, a senior investigator for the Project on Government Oversight watchdog group in Washington, said an additional concern is lax security at government nuclear weapons labs at Los Alamos, N.M., and elsewhere.

In a report issued last fall, the group cited exercises in which mock terrorist teams provided by the U.S. military were able to penetrate defenses at Los Alamos.

He said there is also a problem with what the Energy Department describes as "inventory differences" in which U.S. nuclear labs have been unable to account for missing radioactive matter.

However, in a recent speech in Washington, Los Alamos National Laboratory Director Donald Cobb said the nation's nuclear weapons laboratories are secure.

Whatever the amount or source of radioactive matter, the primary concern about dirty bombs is not simply the casualties at the scene, which would be comparatively light, but the widespread panic and fear.

An attack would be "not very effective as a means of causing fatalities," Richard Meserve, chairman of the federal Nuclear Regulatory Commission, told a Senate Foreign Relations subcommittee last month. "But it could have a psycho-social effect, and terrorists' greatest weapon is fear."

Cobb told the committee that Russia is working to keep tight control of its nuclear weapons but that other sources of radioactive matter were far more accessible.

"Nuclear weapons and weapon-usable materials tend to be focused in military applications under tight government oversight," he said. "Radiological sources are more widespread and have fewer controls."

Zubaydah had earlier warned that Al Qaeda had targeted banks in the northeastern United States for terrorist attack, which prompted an FBI alert last week.

Fleischer was asked Tuesday whether the White House considered Zubaydah to be credible.

"Those are judgments that intelligence experts make based on not only what he says but on other pieces of information that will corroborate information," Fleischer said. "Obviously, his capture is a significant asset to the United States government."

Copyright 2002, Chicago Tribune