Three American diplomats were briefly detained in Russia near a military test site where a mysterious radiation explosion occurred in August, several Russian state news agencies reported.

The U.S. Embassy confirmed the incident, the Interfax news service said, but said that three diplomats had filed the appropriate documents to travel to the area.

The Russian foreign Ministry said diplomats had named another city as their destination and “obviously got lost.”

The report comes just days after the United States said the accident was caused by a nuclear reaction when Russia tried to retrieve a nuclear-powered cruise missile from the Barents sea.

The diplomats were detained on Monday on a train in the city of Severodvinsk, not far from where Russian authorities said they were testing a rocket engine with a nuclear component before the crash.

The diplomats, whom Interfax identified as military attaches, were later released but could be held administratively liable for travel in a restricted military area, the agencies said.

In a statement, the Russian foreign Ministry confirmed that the diplomats were on an official trip, and informed the Russian defense Ministry of their plans.

“Only they announced their intention to visit Arkhangelsk and were on their way to Severodvinsk,” the Ministry said.

“They’re obviously lost. We are ready to provide the us Embassy with a map of Russia, ” the Ministry added.

An explosion at a military training ground in August killed at least five people and caused panic after radiation levels in nearby Severodvinsk jumped 16-fold.

Russian authorities have provided little information about the incident. However, a us diplomat said this week that the accident occurred when Russia tried to retrieve a cruise missile with a nuclear power plant, Burevestnik, from the Barents sea.

“The United States has determined that the explosion at Nyonoksa was the result of a nuclear reaction that occurred during the recovery of a Russian cruise missile with a nuclear power plant,” diplomat Thomas Dinanno said during a speech at the United Nations.

Russia’s plans to install a nuclear-powered cruise missile that could theoretically fly indefinitely were first unveiled by Vladimir Putin during a speech last year. The missile is still being tested, and some weapons experts doubt it can ever be carried out.

The Russian military was trying to retrieve the missile from another failed 2017 test when the accident occurred.

It was not immediately clear whether the diplomats were heading to Nenoxa, a village near the military training ground, at the time they were detained. But the train timetable will mean they were returning from the village when they were arrested around 6pm in Severodvinsk.

Russia has kept a lid on the incident, closing the waters in the White sea to foreign vessels to prevent them from gathering information about the explosion.