Two months after what is widely believed to have been a nuclear test by North Korea, the radioactive particles to prove it have yet to be detected, suggesting the communist state is getting better at hiding the fallout, experts say.

Fifty-five days after North Korea's previous test in 2013, those particles, known as radionuclides, wafted over a monitoring station in Japan, proving what seismic evidence already suggested — that an atomic bomb had been detonated.

Now, 64 days after North Korea announced its latest test and seismic data appeared to confirm it, diplomats and nuclear analysts suspect final technical proof may never materialize.