North Korean nuclear test: Spy agencies scrounge for details

"History would teach us that the North Koreans do like to hide their secret activities and control the message," said David Albright, a private nuclear expert who has visited North Korea and talked with officials about its nuclear program.

A European national security official said the North Koreans were becoming "very effective" at hiding evidence that would offer clues to its nuclear secrets.

A South Korean official knowledgeable about the February 12 test said that most likely the North Koreans dug a test tunnel deeply and sealed it tightly to prevent detection.

"The most plausible point is the structure of the pit was made so that it wasn't a straight line that opened to the outside, but had multiple turns and also many intercepting blockades," he said.

"We need to remember that this is deep in the mountains (where) they tested that are formed of heavy rocks, not out in flat, exposed area," the official said, adding: "We may not find anything."

South Korean, U.S. and European officials all noted that the trace materials sometimes decay rapidly - in the case of highly enriched uranium within a couple of days after an explosion. The longer no traces are found, the less likely that any traces will be found.