Vienna ? 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?

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.

Report notes concerns

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.

Terrorism threat

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.

Atomic sleuthing

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.

• Even conventional forensic clues contaminating a site or sample — hair, fibers, soil — can help.

The results of atomic sleuthing can be “astounding,” Michael R. Carter, a security specialist at California’s Lawrence Livermore National Laboratory, said at the Georgetown session.

Without elaborating, he said U.S. analysts made “definitive” findings in the case of highly enriched uranium seized from smugglers in the ex-Soviet republic of Georgia in 2006, the most recent such weapons-grade case in Hoskins’ files. The material is widely assumed to have come from Russia.

But in many cases pinpoint results would be impossible, because of vast gaps in database information needed to trace fissile material.

“The problem is this kind of data is not shared regularly,” said the IAEA’s Laura Rockwood. Nuclear fuel manufacturers view it as proprietary information. Governments see national security risks in handing it over, particularly to share with states without nuclear weapons.

Lack of cooperation

Last year’s U.S. scientists’ report, by a task force of the American Association for the Advancement of Science and the American Physical Society, urged nuclear weapons states to cooperate more closely on an international database.

Klaus Mayer, a nuclear forensic specialist with the European Commission, doesn’t see that happening.

“There are so many sensitivities involved,” he told The Associated Press by telephone while visiting Georgia. Instead, the Europeans see more promise in a decentralized system of shielded national databases, to be queried in emergencies.

Like the U.S. scientists’ report, the new GAO study said the diminishing ranks of veteran American forensic specialists — less than 50 — must be refilled with newly trained Ph.D. radiochemists and other specialists.

Because the U.S. last tested a nuclear bomb in 1992, “few scientists remain at the national laboratories with hands-on experience in using radiochemistry techniques on debris from a nuclear event and analyzing the results,” it said.

The GAO said Defense, Homeland Security and other U.S. departments must better coordinate programs for producing new nuclear sleuths, and field analytical gear, to be deployed by hazmat-suited scientists at a scene of devastation, must be modernized.

The relevant agencies either concurred in or had no comment on the GAO’s recommendations.

Slow answers

Whatever improvements are made, the assessment by a “CSI: Nuclear” team after an attack can never be cast with absolute certainty — even with a claim of responsibility — and will always take days or weeks to deliver to a president, say those who study the problem.

Cristina Hansell, a scholar at California’s Martin Center for Nonproliferation Studies, figures the Ph.D. detectives might finger an alleged culprit — a surreptitious North Korean bomb, a stolen Russian device, a Pakistani weapon in extremists’ hands — “with 65 to 70 percent likelihood.”

“Are you going to bomb someone else on that basis?” she wondered.

Such questions will come into sharper focus later this year or early in 2010, when Obama plans a global summit on nuclear security, to better mobilize to face what he says will be a “lasting threat” in a terror-ridden world.