By Francois Murphy

VIENNA (Reuters) – The operator of a global network of radioactivity sensors said on Monday its two Russian sites closest to a mysterious explosion on Aug. 8 went offline two days after the blast, raising concern about possible tampering by Russia.

The Russian Defense Ministry, which operates the two stations, did not immediately reply to a request for comment.

Russia’s state nuclear agency Rosatom has acknowledged that nuclear workers were killed in the explosion, which occurred during a rocket engine test near the White Sea in far northern Russia.

The explosion also caused a spike in radiation in a nearby city and prompted a local run on iodine, which is used to reduce the effects of radiation exposure.

Russian authorities have given no official explanation for why the blast triggered the rise in radiation. U.S.-based nuclear experts have said they suspect Russia was testing a nuclear-powered cruise missile vaunted by President Vladimir Putin last year.

“We’re … addressing w/ station operators technical problems experienced at two neighboring stations,” Lassina Zerbo, head of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), said on Twitter overnight.

The CTBTO’s International Monitoring System includes atmospheric sensors that pick up so-called radionuclide particles wafting through the air. Zerbo said data from stations on or near the path of a potential plume of gas from the explosion were still being analyzed.

“COMMUNICATION AND NETWORK ISSUES”

The two Russian monitoring stations nearest the explosion, Dubna and Kirov, stopped transmitting on Aug. 10, and Russian officials told the CTBTO they were having “communication and network issues”, a CTBTO spokeswoman said on Monday.

“We’re awaiting further reports on when the stations and/or the communication system will be restored to full functionality.”

While the CTBTO’s IMS network is global and its stations report data back to CTBTO headquarters in Vienna, those stations are operated by the countries in which they are located.

It is not clear what caused the outage or whether the stations might have been tampered with by Russia, analysts said.

“About 48 hours after the incident in Russia on Aug. 8 these stations stopped transmitting data. I find that to be a curious coincidence,” Daryl Kimball, executive director of the Arms Control Association, a Washington-based think tank.

He and other analysts said any Russian tampering with IMS stations would be a serious matter but it was also likely to be futile as other IMS or national stations could also pick up telltale particles.

“There is no point in what Russia seems to have tried to do. The network of international sensors is too dense for one country withholding data to hide an event,” said Jeffrey Lewis, director of the East Asia Non-Proliferation Program at the Middlebury Institute in California.

The CTBTO’s Zerbo also posted a simulation of the explosion’s possible plume, showing it reaching Dubna and Kirov on Aug. 10 and Aug. 11, two and three days after the explosion.

Rosatom has said the accident, which killed five of its staff, involved “isotope power sources”.

The CTBTO’s IMS comprises more than 300 seismic, hydroacoustic, infrasound and radionuclide stations dotted around the world that together are aimed at detecting and locating a nuclear test anywhere. Its technology can, however, be put to other uses, as in the Russian case.

(Additional reporting by Andrew Osborn in Moscow; Editing by Mark Heinrich)