Surveillance aircraft sweeping Asian skies for a hint of krypton or a sign of strontium have deepened the mystery about North Korea’s claimed nuclear explosion. Was the blast that rocked global politics an atomic fizzle, a mini-test or simply the thunder from hundreds of tons of TNT?

A senior U.S. administration official said Friday that one of a number of tests of samples was consistent with a nuclear explosion, but no definitive conclusion had been reached.

Earlier in the day, a U.S. government intelligence official said initial air sampling in the area Tuesday by a specialized U.S. WC-135 aircraft found no evidence of the radioactive particles produced by successful nuclear explosions. Chinese and Japanese monitoring also has detected no telltale evidence.

Both U.S. officials spoke on condition of anonymity because of the sensitive nature of the information.

Experts outside government played down the significance of the largely negative findings, and dismissed the idea the test might have been a fake staged with conventional explosives.

Studying the few scraps of information available, specialists did warn that prolonged uncertainty about just what happened Monday inside a North Korean mountain could lead to misperceptions and a potentially dangerous backlash.

“If the North Koreans fear that we perceive it didn’t work, will they fear American military action more than before?” asked George Perkovich, of Washington’s Carnegie Endowment for International Peace. “And in that case, do they rush into another test, or what?”

Estimates of the power of the blast, derived from seismic readings worldwide, vary widely. The French think it may have been as small as the equivalent of 500 tons of TNT – barely 3 percent of the power of the bomb the United States dropped on Hiroshima in 1945. But the Russians project it may have approached Hiroshima size.

The smaller scale might indicate a problem in the complex detonation process, presumably of an “implosion” model using a plutonium core encased in a sphere of conventional explosives.

“It probably misfired. I would call it a partial failure,” said physicist David Albright, whose Washington-based Institute for Science and International Security closely tracks North Korean nuclear developments.

Experts also differ on whether the explosion, believed to have been carried out in a sealed horizontal shaft deep underground, would have vented radioactive particles into the atmosphere.

The North Koreans said it hadn’t.