Moscow (CNN) An official state of mourning has been declared in the Russian city of Sarov. Last Thursday, five nuclear specialists employed by Rosatom, Russia's state atomic energy corporation, were killed in a blast at a military test site in northern Russia, not far from the port of Severodvinsk.

According to the official account, the elite scientists killed in the accident -- Alexey Vyushin, Yevgeny Koratayev, Vyacheslav Lipshev, Sergey Pichugin and Vladislav Yanovsky -- were killed during tests on a liquid propulsion system involving isotopes.

Sarov, known during the Cold War as Arzamas-16, is one of Russia's secret cities. Closed to foreigners and accessible only by special permit, Sarov is the rough equivalent to Los Alamos, New Mexico, one of the birthplaces of US nuclear weapons design.

In other words, the test most likely had some nuclear dimension. And the reflexive secrecy of the government of Russian President Vladimir Putin has only further fueled speculation about the cause of the accident.

Here's what we know: authorities in northern Russia detected a brief rise in radiation levels following an explosion at a military training ground there, Russian state news agency TASS reported Thursday.

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