By CHARLES J. HANLEY
AP Special Correspondent
VIENNA (AP)—If the unthinkable happened, would we be left on the day after, as radioactive dust settled, with the unknowable?
If a terrorist nuclear bomb destroyed the heart of a great city, how would we know who did it, with what? Mideast fanatics with a device improvised from stolen uranium? A weapon smuggled in by a rogue regime? A hijacked U.S. bomb?
Where do you strike back? How do you head off another attack?
U.S. President Barack Obama calls nuclear terrorism "the most immediate and extreme threat to global security." It's an unthinkable that's being thought about daily in classified corners of world capitals.
But knowledgeable scientists and the investigators behind a new U.S. government report say the American nuclear establishment needs more specialists and more background data on possible bomb sources to do the detective job that awaits on that day after.
"I don't believe the intelligence community is ready for the challenge," said Rolf Mowatt-Larssen, who worked for years as a U.S. intelligence leader on weapons of mass destruction.
The concerns are evident in the June 1 government report, an unclassified version of a classified assessment by the Government Accountability Office (GAO), and in an earlier study by major U.S. scientific organizations.
They say an aging, shrinking corps of nuclear forensic experts and U.S. analytical facilities would be badly stretched if a city-leveling nuclear weapon or a "dirty bomb" spreading radioactivity was detonated in the United States.
The scientists also said international databases cataloging characteristics of nuclear materials worldwide, essential for tracing clues in such an event, are currently "not nearly extensive or usable enough."
"If you have the reference data, you can identify the origin. It's just like a fingerprint database," explained Richard Hoskins, a security expert at the International Atomic Energy Agency, in an interview at the IAEA's Vienna headquarters.
Hoskins keeps a global watch on nuclear smuggling. His IAEA database counts 1,646 incidents of trafficking, theft or loss of nuclear materials since 1995, including 18 involving plutonium or highly enriched uranium, nuclear bomb fuels.
No case involved enough material to build a bomb, and Hoskins said his agency detects no strong evidence of a terrorist network, rather than opportunistic thieves, behind any incident. But they don't know what they don't know, he stressed.
"We know that the size of the problem" — both successful and failed attempts — "is probably substantially larger than the number we have," he said.
And at least one terror group is known to aspire to nuclear status, noted another Vienna-based authority, Roger Howsley, head of the World Institute for Nuclear Security, a newly formed, U.S.-supported body to advise on safeguarding nuclear facilities.
"Al Qaida has said it would if it could," he said.
Terrorists face daunting challenges in trying to steal a usable bomb, or build an effective model if they obtain bomb material, experts say.
But "even the minute chance that terrorists might have that ability changes the equation dramatically," Mowatt-Larssen, an ex-CIA official and former U.S. Energy Department intelligence chief, said at a recent "Post-Nuclear Event" discussion at Washington's Georgetown University.
To prevent equation-altering breakthroughs, nuclear forensics is deployed "pre-detonation," to try to trace material seized from traffickers back to the source, to plug leaks at vulnerable nuclear facilities.
With sophisticated equipment and training, chemists and physicists at U.S. national laboratories and elsewhere can learn much from analyzing a few grams of fissile material. For example:
—The ratio of isotopes in natural uranium — of U-238, U-235 and U-234 — varies from place to place and can tip investigators to where a sample of uranium was mined.
—Plutonium's isotopes vary according to the reactor that made it. Bomb-makers Russia, North Korea, Pakistan and Israel, for example, all have different kinds of plutonium-producing reactors.
—The grain size and shape of bomb material can pinpoint a manufacturing process.