Three U.S. diplomats were stopped Monday on a train from Nyonoksa to Severodvisnk, in Northern Russia, and held for an unknown amount of time. They were then released after a document check, according to CBS News.

Nyonoksa is near the alleged accidental nuclear missile explosion site, that left at least seven people dead, in the beginning of August, 2019. Business Insider, Vice, Bloomberg, LA Times, and several other media sources claim that Russia is actively trying to cover the explosion up from this past summer.

An unnamed Rosatom State Nuclear Energy Corporation official told state news agency TASS that the explosion happened during testing of a missile at sea and that the blast, which occurred when the fuel caught fire, had knocked several people off of a platform and into the water.

Multiple injured people were sent to the hospital, the doctors were not informed of the possibility of radiation exposure, the doctors were sent afterwards to a hospital in Moscow for radiation testing, and everyone was forced to sign non-disclosure agreements. (RELATED: Warm And Glowing: Team Of Scientists Creates ‘Atomik’ Chernobyl Vodka)

“This event has been hugely embarrassing for the authorities, and they’re doing almost everything they can to minimize its significance and suppress information about it,” Joshua Pollack, an expert on nuclear weapons at the Middlebury Institute for International Studies, told Vice. “They’re running in the opposite direction, trying to hide the details and obfuscate.”

U.S. intelligence officials and arms control expert Jeffery Lewis said it was “likely” a failed test of the SSC-X-9 Skyfall missile, known in Russia as the 9M730 Burevestnik. President Vladimir Putin boasted in a 2018 speech about a new, experimental nuclear-powered cruise missile with virtually unlimited range and the ability to penetrate U.S. missile defense systems.

An August 8 image from @planetlabs showing the Serebryanka, a nuclear fuel carrier, near a missile test site in Russia, where an explosion and fire broke out earlier. The ship’s presence may be related to the testing of a nuclear-powered cruise missile. pic.twitter.com/QhdxuDC91w — Jeffrey Lewis (@ArmsControlWonk) August 9, 2019

After the explosion, the city administration in Severodvinsk said the radiation levels there rose to 2 microsieverts per hour — approximately 20 times the area’s average reading — for about 30 minutes. It then returned to the area’s average natural level of 0.1 microsieverts per hour. Nyonoksa residents were asked to leave the village for several hours. The order was quickly rescinded by the military, who said they canceled the activities at the range that had warranted the initial evacuation order.

President Donald Trump made a brief statement about it on twitter. (RELATED: Russia Violates Critical Nuclear Treaty, NATO Speaks Out)

The United States is learning much from the failed missile explosion in Russia. We have similar, though more advanced, technology. The Russian “Skyfall” explosion has people worried about the air around the facility, and far beyond. Not good! — Donald J. Trump (@realDonaldTrump) August 12, 2019

The entire area between Nyonoksa and Severodvisnk is now restricted to foreigners without proper permits. According to Russia’s Foreign Ministry, the diplomats declared the city of Arkhangelsk as their destination, which is about 25 miles from Severodvinsk. “Only, they said their intention was to visit Arkhangelsk and they ended up en route to Severodvinsk,” the ministry said. “They obviously got lost. We are ready to give the US embassy a map of Russia,” the ministry added.

The diplomats could face administrative charges for traveling in a restricted military area, agencies reported.