The explosion that killed five Russian scientists during a rocket engine test earlier this month was followed by a second blast two hours later, the likely source of a spike in radiation, Norway’s nuclear test-ban monitor said on Friday.

The second explosion, detected only by infrasonic air pressure sensors and not by the seismic monitors that pick up movements in the ground, was likely from an airborne rocket powered by radioactive fuel, the Norsar agency said.

Russia’s state nuclear agency, Rosatom, on Aug. 10 said the accident involved “isotope power sources” but did not give further details.

Russia’s Ministry of Defence did not immediately respond to a request for comment when contacted by Reuters on Friday.

Rosatom has acknowledged that five of its workers and two military personnel were killed in the Aug. 8 explosion near the Russian navy’s range in Nyonoksa on the White Sea. 

It was followed by a brief rise in radiation levels in nearby Severodvinsk, but Russian authorities insisted the recorded levels didn’t pose any danger to local residents.

Russian officials’ changing and contradictory accounts of the incident drew comparisons to Soviet attempts to cover up the 1986 explosion and fire at the Chornobyl nuclear power plant in Ukraine, the world’s worst nuclear disaster. Russia’s Defence Ministry initially said background radiation remained normal, while the state weather agency said radiation levels had risen.

Norway’s DSA nuclear safety authority said on Aug. 15 it had found tiny amounts of radioactive iodine near Norway’s Arctic border with Russia, although it could not say whether it was linked to the Russian accident.

Norsar’s detection of a second blast was first reported by Norwegian newspaper Aftenposten on Friday.

“We registered two explosions, of which the last one coincided in time with the reported increase in radiation,” Norsar chief executive Anne Stroemmen Lycke told Reuters, while adding that this likely came from the rocket’s fuel.

“Both blasts were registered on our infrasound system. The first was also picked up by seismology,” she added.