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Russia’s Federal Security Service said Sunday it detained a teenager suspected of planning an attack at a popular Moscow park and museum, a day after the US Embassy warned of a “credible threat of a mass casualty event” there.

Getting into it: The US Embassy issued a security alert Saturday about the Tsaritsyno Museum Reserve, a palace and park complex about 10 miles south of the Kremlin, telling people to stay away and saying it was coordinating with Russian authorities. The German Embassy put out a similar warning. Police closed and cordoned off the park Saturday evening, evacuated visitors and canceled several events.

The FSB said the suspect, a Russian citizen and a minor whose name wasn’t released, had posted photos on TikTok of himself in a balaclava and tactical gear holding what looked like an automatic weapon, along with pictures taken at Tsaritsyno captioned “I found a place.” A search of his home turned up the clothing from the photos and airsoft guns, and his phone showed a subscription to a neo-Nazi Telegram channel and a large number of searches about past terrorist attacks and mass killings, according to the FSB. Russia’s Investigative Committee is now weighing charges.

In a statement, the FSB said, “We are grateful to the American side for providing the information.”

The US has a long-standing policy of warning other countries and Americans abroad about threats it learns of, but Moscow hasn’t always welcomed them. In March 2024, the US Embassy warned of plans to target large gatherings in Moscow, and Russian President Vladimir Putin brushed it off as a provocation. Weeks later, gunmen from an ISIS affiliate attacked the Crocus City Hall concert venue outside Moscow, killing more than 140 people.

During Trump’s first term in 2017, Putin personally thanked Trump after a US tip helped Russian authorities stop a planned attack on a cathedral in St. Petersburg.