U.S. Intelligence officials are searching for the cause of an explosion that occurred off the coast of northern Russia last week. The weapon was a nuclear-propelled missile that Russian President Vladimir Putin has called the “invincible” centerpiece of the new Russian-United States arms race.

According to reports, the blast left seven dead including five scientists. The Russian government’s slow response to the explosion has worried neighboring towns and officials within the U.S. government and in Europe. 

The blast was set off near Nenoksa Missile Test Site and released a substantial amount of radioactive material into the atmosphere. On Sunday night, the research institute where the five dead scientists worked confirmed that the blast was from a nuclear reactor in the White Sea. 

American investigators suspect that a missile that NATO calls SSC-X-9 was the cause. Putin has previously bragged that his new weapon could reach the opposite side of the world powered by a nuclear reactor.  

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In recent reports from the Pentagon, the new missile has been cited as a potential threat. These would virtually unstoppable as they fly at low altitudes and anti-ballistic missile systems on the west coast of the United States could not shoot them down. 

Many experts have called Putin’s efforts to build an undetectable long range missile as an effort to revitalize the arms race between the two countries that took place from the 1950’s until the Soviet Union dissolved.  

Putin has said in previous statements that the nuclear propelled weapon is an answer to America’s “global strike” weapons.