Plutonium or uranium: Which was the fissile material North Korea used in last month's nuclear test? We may never know, says a nuclear watchdog agency. The Comprehensive Nuclear Test Ban Treaty Organization hasn't found any "smoking gun" radioactive traces yet, and acknowledges that "it is very unlikely that we will register anything at this late stage." Reuters says the admission may mean that North Korea was able to successfully contain all nuclear traces that could have allowed for outside analysis. The CTBTO expressed confidence in the abilities of its system; it monitors nuclear tests around the globe via 270 stations.