At a June 1 news conference, the Justice Department said the alleged al Qaeda associate hoped to attack Americans by detonating "uranium wrapped with explosives" in order to spread radioactivity.
But uranium's extremely low radioactivity is harmless compared with high-radiation materials — such as cesium and cobalt isotopes used in medicine and industry that experts see as potential dirty bomb fuels.
"I used a 20-pound brick of uranium as a doorstop in my office," American nuclear physicist Peter D. Zimmerman, of King's College in London, said to illustrate the point.
Zimmerman, co-author of an expert analysis of dirty bombs for the U.S. National Defense University, said last week's government announcement was "extremely disturbing — because you cannot make a radiological dispersal device with uranium. There is just no significant radiation hazard."
Other specialists agreed. "It's the equivalent of blowing up lead," said physicist Ivan Oelrich of the Federation of American Scientists.
When Padilla was arrested in June 2002, after returning to Chicago from Afghanistan and Pakistan, Attorney General John Ashcroft said the ex-Chicago gang member and Muslim convert had planned a dirty bomb that could "cause mass death and injury." Washington, D.C., was the likely target, his department said.
But it wasn't until Deputy Attorney General James Comey's briefing for reporters last week that authorities said Padilla had uranium in mind for his radiological dispersal device, or RDD, the technical term for such a weapon. Comey said the detainee disclosed he'd also been sent to set off natural gas explosions in U.S. apartment buildings.
"Just saying the word `uranium,' the public automatically assumes, `Oh, it sounds bad,"' said physicist Charles Ferguson of the Washington office of California's Monterey Institute of International Studies. He co-authored one of the most detailed reports on the dirty-bomb threat.
Those studying the RDD potential envision a combination of explosives with a lethal radioisotope, such as cesium-137, diverted from use in cancer radiotherapy, for example, or from machines that irradiate food. Particularly if in powder form, it could spew intense radioactivity over a section of a city, making it uninhabitable.
Radiation from uranium, on the other hand, is billions of times less intense than that of cesium-137, cobalt-60 and other radioisotopes. It's not radioactivity but another property of uranium — its ability in some forms to sustain atomic chain reactions — that makes it a fuel for nuclear power and bombs.
The Justice Department didn't respond directly when asked this week whether it had consulted with experts and knew that uranium wouldn't make a dirty bomb.
Instead, spokesman Mark Corallo said Padilla's statements, in view of his al Qaeda links, made clear that he was "willing to cause devastating harm to innocent Americans."
Padilla has been held by the U.S. military since 2002 as an enemy combatant, without charge and with little access to lawyers. The Bush administration has been criticized for denying a U.S. citizen normal access to the courts. The Supreme Court is considering whether the government, in defending against terrorism, has such power.
Padilla's lawyer, Donna Newman, said Wednesday of the dirty-bomb allegation that U.S. authorities "should have known that this was nonsense."
"When they frightened everybody, what were they trying to do, if they knew better? To show the administration is on top of things?" she asked.
She wants the government to attempt to indict and try her client. "Maybe the problem is the evidence is so weak, it's laughable," she said.
Comey said the news conference was called "to help people understand the nature of the threat" Padilla posed.
Based on what he said were Padilla's admissions to interrogators, he described a "highly trained al Qaeda soldier" who accepted an assignment to blow up U.S. apartment buildings, and "planned to do even more by detonating a radiological device, a dirty bomb, in this country."
Spokesman Corallo reaffirmed this week that it was Padilla who said uranium would be used.
"If that's what he planned," physicist Oelrich said of Padilla, "it shows he doesn't know what he's talking about and hasn't done even rudimentary homework."
He wasn't the only one, according to a Justice Department summary of interrogations.
It said Abu Zubaydah, a top al Qaeda lieutenant now in U.S. custody, also envisioned a uranium device when urging Padilla to mount a U.S. attack. At another point, however, the summary said Zubaydah told Padilla the dirty bomb was "not as easy to do as they thought."
Padilla claims "he was never really planning to go through with" any of the terrorist assignment, Comey told reporters.
As a heavy metal, like lead, uranium poses one health risk: If ingested or inhaled, it can damage kidneys or other organs. But unlike radioisotopes, byproducts of nuclear reactors, uranium doesn't emit penetrating gamma rays that cause acute radiation poisoning. Instead, it slowly radiates weak alpha particles, which don't even penetrate skin.
"Granted, it (uranium) could have a psychological effect" because of unfounded fears, said physicist Ferguson. But he said a government information campaign should quell any panic if such a weapon appeared.