Panetta said U.S. technical experts are still assessing available data to learn more about the underground explosion.
North Korea confirmed the test after international monitors recorded seismic activity consistent with a powerful underground explosion at 11:57 (02:57 GMT) on Tuesday.
BBC: North Korea nuclear test branded 'serious threat' to US
"Highly enriched uranium is something that degrades quickly, so you would have to collect within a 24-hour period, " especially because the traces from an underground explosion will be minimal, he said.
The man, who had previously failed the test to qualify as an inspector, revealed the deception in March, 2010 a month before the massive underground explosion that killed the 29 miners.
FORBES: New Evidence of Deceit and Deception in Mine Disaster that Killed 29
Nuclear test monitors in Vienna say the underground explosion had double the force of the last test, in 2009, despite the use of a device said by the North to be smaller.
Nuclear test monitors based in Vienna say the underground explosion had double the force of the last test, in 2009, despite the use of a device said by the North to be smaller.
"Well No. 4 in this area was being drilled at this site here when suddenly an underground steam explosion managed to get its way to the surface, " he explains.
Shatter cones are surface features with distinctive wavy patterns that are known to be created only by the tremendous force of a meteorite impact or an underground nuclear explosion.
Pyongyang is angry about ongoing U.S.-South Korean military drills and about new U.N. sanctions, issued over the North's December long-range rocket launch, which the U.N. called a cover for a banned missile test, and its third underground nuclear explosion, conducted Feb. 12.
Iran has denied foreign media reports of a major explosion at one of its underground uranium enrichment sites.
In 1993, an early-morning explosion in Managua below the workshop of a Basque mechanic revealed an underground bunker stuffed with weapons, forged passports, and documents, including plans for past kidnappings to raise money, notably that of a Brazilian industrialist, Abilio Diniz, in 1989.
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