WASHINGTON -- U.S. scientists and intelligence experts are engaged in detective work with international implications as they sort through data -- still coming in -- on the underground detonation in North Korea on Monday.
The White House and other officials said the final answer about whether the communist government exploded a nuclear device may still be several days away.
An air sampling taken after North Korea's claimed nuclear test detected radioactivity consistent with an atomic explosion, Bush administration and congressional officials said Friday night. They said no final determination had been made about the nature of last weekend's mystery-shrouded blast.
One U.S. government official said intelligence officials assigned an 80 percent probability that the North Korean explosion was a nuclear detonation, based on the air sample collected Wednesday. The official said it appeared highly unlikely that the sample of radioactive material was produced by any other source, including a nuclear power reactor.
Officials spoke on condition of anonymity given the tense situation with North Korea.
"The betting is that this was an attempt at a nuclear test that failed," a senior administration official said. "We don't think they were trying to fake a nuclear test, but it may have been a nuclear fizzle."
Other tests from air sampling conducted by the United States, Japan and China found no trace of radioactive material.
If North Korea did conduct a nuclear test, there could be any number of reasons for negative tests. For example, it could be that the device -- detonated under a mountain -- simply didn't vent radiological material in an appreciable, detectable way.
At least a half-dozen U.S. agencies are involved in the detective work. In the United States alone, the key participants include the Central Intelligence Agency, Air Force, Defense Intelligence Agency, Energy Department, National Security Agency and National Geospatial-Intelligence Agency, which studies satellite imagery.
All of them answer to the national intelligence director's mission manager for North Korea, Joseph DeTrani.
Surprising some, North Korea's claimed nuclear detonation -- its first -- was not very large. Experts in and out of government had expected North Korea to detonate a device that was at least several kilotons; each kiloton is equal to the force produced by 1,000 tons of TNT.
The device tested Monday was less than a kiloton -- perhaps 500 tons or less -- raising questions about whether it was a botched effort, buried deep underground and well-muffled, or a conventional explosion that the North Koreans tried to portray as a nuclear test.
The small size also made it more difficult to detect, U.S. officials have said.
Nuclear tests conducted in the atmosphere create a double pulse of light that U.S. satellites are able to detect, buttressed by air samples.
But with underground tests like this one, analysts and scientists are left to analyze the air samples picked up from the WC-135, the Constant Phoenix atmospheric collection aircraft. The Air Force jet has a specially configured sensor suite to collect information from radioactive clouds in real time after nuclear tests.
The experts are also looking over seismic readings gathered by the U.S. Geological Survey and the Air Force Technical Applications Center, the military organization that's primarily responsible for reporting on nuclear events. Headquartered at Florida's Patrick Air Force Base, the center operates a worldwide network of nuclear event detection sensors, which includes satellites and a ground-based seismic network.
The trick is that the seismic signal for a nuclear detonation is not necessarily proportional to the explosive device. It depends on factors such as how tightly the device was secured in the earth and the density of the surrounding rock. The signal may give an incomplete picture.
Imagery analysts are also using pictures from satellites to detect any potential changes to the test site. There may be a range of other types of satellite imagery available to analysts, such as radar that detects even small changes in the earth's surface. But it's not clear how useful that information will be in reaching a conclusion.
The U.S. may also be seeking other clues from defectors and other agents. And the eavesdroppers at the National Security Agency are also likely to be listening for communications that could provide helpful information.
All of it comes together to help analysts reach a conclusion -- in theory.
As the work is going on, intelligence officials are also paying close attention to the possibility that North Korea might test a second device sometime soon, as other countries have in the past. U.S. officials said government experts haven't ruled out the possibility that North Korea would take such a step, but the U.S. hasn't detected any immediate test preparations.
Associated Press writers Robert Burns and Anne Gearan contributed to this report.