The US and China had both confirmed earlier that no radioactive particulate had been found in air samples, casting some suspicion on the claims that a nuclear test had been conducted by North Korea. There is new information that the US has detected radioactive gas, however (probably xenon) and there is now speculation that this was a fizzle.
The official, who spoke on condition of anonymity, cautioned that the administration has not made a definitive conclusion about the nature of the explosion.
"The betting is that this was an attempt at a nuclear test that failed," the official said. "We don't think they were trying to fake a nuclear test, but it may have been a nuclear fizzle — an effort that failed." The official spoke on condition of anonymity because of the sensitive nature of the information.
The official said the test measures a type of gas. It is one of a number of analyses conducted this week, which have not provided clarity about what North Korea detonated on Monday.
Earlier Friday, results from another test disclosed Friday — an initial air sampling — showed no evidence of radioactive particles that would be expected from a successful nuclear detonation, a U.S. government intelligence official said.
Xenon is very slippery and can get out pretty easily. (I was dosed by radioactive xenon a long time back – it was minor, however). I'm only guessing that's what they detected, however.