Reports of a second blast from a deadly Russian rocket engine test may be wrong, and the signals could stem from unrelated mining activity, the Norwegian monitor that first presented the double explosion theory said.

Norsar, Norway's nuclear test-ban monitor, last week said an Aug. 8 explosion that killed five Russian scientists was followed by a second blast two hours later, and that this was the likely source of a spike in radiation.

The second explosion was detected by infrasonic air pressure sensors in the Norwegian town of Bardufoss, but further analysis, taking in additional data from Norway and Finland, pointed to a different explanation, Norsar said on its website.