Four of Russia’s nuclear monitors went silent in the days following a mysterious explosion that led to radiation spikes in the country’s Northwest region, the Wall Street Journal reported, fueling suspicions that Moscow may have tampered with them.

The radiation detection stations resumed reporting data on Tuesday, according to a report from the Associated Press.

Russian officials have confirmed at least five nuclear engineers died during a rocket test involving “isotope power sources” in the White Sea, leading to international speculation that the accident involved a cruise missile that runs on nuclear power.

Four international stations designed to monitor nuclear activity have detected “an event coinciding with the [August 8] explosion in Nyonoksa, Russia,” the world’s main nuclear test-ban body said.

The Comprehensive Nuclear-Test-Ban Treaty Organization is an independent body that watches for nuclear weapons testing violations, and it maintains more than 300 stations around the world. Both the Russian and the United States are signatories of the treaty.

The Dubna and Kirov stations, which are closest to Russia’s Arkhangelsk , where the blast took place, told the CTBTO they were experiencing “communication and network issues” shortly after the explosion, its chief Lassina Zerbo told The Wall Street Journal.

“We are pending further reports on when the stations or the communication system will be restored to full functionality,” Zerbo was quoted as saying Sunday.

Late on Tuesday, Sergei Rybakov, Russia’s foreign minister seemed to acknowledge the apparent blackout, telling Interfax that Russia’s transmission of data from radiation stations to the Vienna-based CTBTO was voluntary, and in any case was not subject to the organization’s consideration.

The CTBTO’s stations “measure the atmosphere for radioactive particles,” an official with the organization told CNN, adding that “only these measurements can give a clear indication as to whether an explosion detected by the other methods was actually nuclear or not.”

Daryl Kimball, the head of the Arms Control Association NGO, called the data disruption “a very odd coincidence.”

“It is probably because they want to obscure the technical details of the missile-propulsion system they are trying and failing to develop,” The Wall Street Journal quoted Kimball as saying. “But this is not a legitimate reason to cut off test-ban monitoring data transmissions.”

The four stations may have intentionally stopped transmitting signals to hide the composition of isotopes involved in the accident from other countries, an unnamed nuclear industry source told the Znak.com news website later on Tuesday.

The mysterious disruption to the radionuclide detection stations came as Russian officials have given conflicting accounts about the level of radiation released in the rocket test explosion.

The Russian Defense Ministry initially said no radiation had been released, although the city administration in Severodvinsk ­– a port city of 183,000 located about 20 kilometers away from were the accident occurred – reported a brief rise in radiation levels on the day of the accident.

The Defense Ministry sought to censor that report, but by the following Tuesday, Russia’s state weather agency Rosgidromet said that radiation levels had spiked by four to 16 times beyond natural background levels in the city.

On the next day, Russian officials announced that Nyonoksa – the village nearest the accident site – would be evacuated for reasons they refused to spell out. But later that same day, Igor Orlov, governor of the Arkhangelsk region, told the Interfax new agency that the evacuation order was “complete nonsense.”