Three U.S. diplomats were removed from a train and detained Monday near a mysterious military site in northwestern Russia where a deadly explosion and radiation leak took place back in August, according to officials.

Russia's Interfax news agency reported that the three U.S. diplomats were removed from a train when it arrived in Severodvinsk, a city of 183,000.

The U.S. State Department confirmed to Fox News the diplomats were taken off the train and were in the country on official travel after properly filing travel with the Russian Defense Ministry.

“The American diplomats were on official travel and had properly notified Russian authorities of their travel," a State Department spokesperson told Fox News.

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Russian broadcaster REN-TV reported the three Americans did not present documents for staying in Severodvinsk, which is a city located in a list of territories that are under regulation for visits by foreign citizens.

The group of Americans was detained after Russian authorities conducted a check of passengers on the train when it arrived in the city, according to REN-TV.

Interfax reported the U.S. diplomats had been let go, but are regarded to have broken Russian law. Officials have not yet released any additional information.

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The port city of Severodvinsk is located near the military shooting range in Nyonoksa, in the country's far northern Arkhangelsk region.

On Aug. 8, five nuclear workers were killed during a rocket engine test near the White Sea in an area considered part of the testing range. Russia's Defense Ministry initially said the blast killed two people and injured six, but the state-controlled nuclear agency, Rosatom, later disclosed that the explosion killed five of its workers and injured three others. Rosatom said the explosion occurred while engineers were testing "a nuclear isotope power source" for a rocket and were thrown into the sea by the explosion.

A Russian newspaper later reported that two of the patients that were injured in the blast died from radiation sickness before they could be taken to Moscow for treatment.

The mysterious explosion was followed by a brief rise in radiation levels in nearby Severodvinsk, but the authorities insisted the recorded levels didn't pose any danger to local residents. Russia's state weather agency, Rosgidromet, said it believed radiation levels had risen up to 16 times after the accident when a cloud of radioactive gases drifted across the area in the wake of the blast.

Weeks later, the weather agency said that radioactive isotopes were been discovered in test samples.

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Russian officials' changing and contradictory accounts of the incident have drawn comparisons to Soviet attempts to cover up the 1986 explosion and fire at the Chernobyl nuclear power plant in Ukraine, the world's worst nuclear disaster.

The Russian Defense Ministry at first denied any radiation leak in the incident, even as authorities in nearby Severodvinsk reported a brief rise in radiation levels and advised residents to stay indoors and close the windows.

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President Vladimir Putin has praised the victims of the disaster, saying they were doing "very important work for the nation's security," but kept mum on what type of weapon they were testing. U.S. defense officials and outside observers believe it was a missile Russia calls the 9M730 Burevestnik. The NATO alliance has designated it the SSC-X-9 Skyfall, which was first revealed by Putin in March 2018 along with other doomsday weapons.