CBS News National Security Correspondent David Martin reports Abu Zubaydah, who until his capture March 28 was bin Laden's chief of operations, has told interrogators al Qaeda knows how to build a so-called dirty bomb that would spew radiation into the atmosphere. He also said al Qaeda knows how it could be smuggled into the United States.
On Friday, the FBI issued a public warning to 1,200 banks in 12 Northeastern states to be on heightened security based on a claim by Zubaydah that al Qaeda is planning attacks on financial institutions in the northeast.
The FBI said Monday the warning remains in effect although the government has no new information that would substantiate reports of any specific threats or plots.
The public alert was being "constantly evaluated" and FBI agents were investigating any potential leads before senior U.S. officials will decide whether to cancel the warning, FBI spokesman Bill Carter said. He could not say how long that might take.
No warning was issued about a dirty bomb, apparently because Zubaydah did not say al Qaeda actually has one. But it is certainly possible.
Such a weapon — also called a radiological dispersal device — would use conventional explosives to spread industrial or medical-grade radioactive material in a populated area to cause widespread fear of exposure.
They are not thought to be difficult to build. Acquiring enough radioactive material to do harm is regarded as the greatest challenge for terrorists.
A radiological device detonated by terrorists would require evacuation and decontamination of the immediate area and disrupt the local economy, officials from U.S. nuclear laboratories said at a recent Senate committee hearing. Hospitals would be overrun by worried people from the affected area.
Depending on factors ranging from the bomb's construction to wind direction on the day such a weapon was used, a potent dirty bomb could kill a few people quickly if they were exposed to enough radiation, officials said. Others would face a greater likelihood of developing cancers later in life.
Severe contamination could require that buildings be razed, and the economic fallout could reach billions of dollars in a big city, officials said. An orderly evacuation would limit the population's exposure to radioactive materials, and health effects would be minimal as long as victims avoided the contaminated area.
Much of the U.S. government's thinking on the subject is theoretical, because no one has detonated a radiological weapon.
They do exist. In 1995, separatists from Russia's embattled Chechnya region announced they had placed Cesium-137 in a Moscow park; it was recovered by authorities. The Chechens, who are believed now to have links to al Qaeda, threatened to covertly release additional materials.
Officials don't know whether Zubaydah is telling the truth or bragging -- or both. Although he was badly wounded during his apprehension and is now being subjected to hostile interrogation, one U.S. official says he is a very tough customer.
The same can be said for most of the other al Qaeda terrorists now in U.S. custody, who have been well trained in how to resist interrogation, officials tell CBS News.
An English translation of an al Qaeda training manual says "the brother should not disclose any information, no matter how insignificant he might think it is." His best hope, even under torture, the manual advises, lies in "executing the security plan -- or cover story -- that was agreed upon prior to ... the operation and not deviating from it."
Most of the prisoners held at Guantanamo Bay, Cuba, seem to be going by the book. None of them has given up information that has allowed the U.S. to break up a terrorist cell or plot, and only 6 out of 300 have admitted to crimes which could be prosecuted by a military tribunal.