U.S. officials and independent experts say North Korea appears to have taken unusual steps to conceal details about the nuclear weapon it tested in February, fueling suspicions that its scientists shifted to a bomb design that uses highly enriched uranium as the core.

At least two separate analyses of the Feb. 12 detonation confirmed that the effects of the blast were remarkably well contained, with few radioactive traces escaping into the atmosphere — where they could be detected — according to U.S. officials and weapons experts who have studied the data…

In the days following the detonation, U.S. and South Korean sensors failed to detect even a trace of the usual radioactive gases in any of the 120 monitoring stations along the border and downwind from the test site, the officials said. A Japanese aircraft recorded a brief spike of one radioactive isotope, xenon-133, but it was seen as inconclusive, the analysts said. Xenon-133 is released during nuclear weapons tests but also given off by nuclear power plants.