Scientists are still trying to determine what caused a blast in North Korea that triggered seismic monitors around the world.
North Korea claims it carried out a successful nuclear test early Monday, but seismic and satellite data have indicated the explosion was relatively small for a nuclear detonation and may have been a failure or only partial success.
Some scientists say data indicate the explosion was less than the equivalent of 1,000 tons (one kiloton) of high-explosives. Analysts have cast doubt on early Russian reports that the blast was much larger.
U.S. intelligence officials say the first detonations of aspiring nuclear powers historically have been the equivalent of several kilotons of explosive force.
Scientists say the North Korean test may have achieved only a partial detonation, and it may be days before radiation detectors sampling air and water near North Korea can provide more data.
A White House spokesman said finding more conclusive evidence is so difficult that outside scientists "may never know" exactly what happened.
Some information for this report was provided by AP and Reuters.