The minor had posted photographs of himself on TikTok wearing tactical clothing and a balaclava, holding an object that looked like an automatic weapon, as well as photographs taken at Moscow's Tsaritsyno Museum Reserve with the English-language caption: "I found a place," the FSB said.
"We are grateful to the American side for providing the information," the security service said in its statement.
It said officials searching the minor's home found the clothes and accessories seen in the social media posts while his phone showed a subscription to a neo-Nazi channel on the app Telegram and "a significant number of searches concerning terrorist attacks, mass killings and other acts of violence," according to Interfax.
Russia's Investigative Committee is considering what legal or procedural action to take, the FSB said.
Tsaritsyno, a palace museum and park complex in southern Moscow, was temporarily closed on Saturday following the US tip-off.
The German embassy in Moscow also issued a warning to avoid Tsaritsyno.
The Tsaritsyno historical estate was originally planned and partially constructed by Catherine the Great as a country residence on the outskirts of Moscow.
The park and its historical buildings were restored approximately 20 years ago.
with DPA
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