The U.S. Embassy in Moscow warned on Saturday of a “credible” security threat that could cause mass casualties at the Tsaritsyno Museum-Reserve, urging people to avoid the area and large gatherings.

The embassy said Washington was coordinating with Russian authorities. Russia’s Federal Security Service confirmed it received U.S. information about preparations for a possible attack and said it was taking necessary measures.

Russian outlet Baza reported that the park had been closed to visitors, with only exits permitted.

The embassy did not identify the nature of the threat or those potentially responsible, and advised people in Moscow to remain vigilant.