Mystery swirled around a U.S. military transport aircraft that on Tuesday landed in Moscow, according to tracking data.

It was not known who was on board the plane and why it was in Russia. When asked, Kremlin spokesman Dmitry Peskov said he did not have information about the aircraft.

U.S. military officials, the White House, the U.S. embassy in Moscow and Russian embassy in Washington did not immediately respond to questions about the flight’s mission.

Data from FlightRadar24 tracking site showed that a Boeing C-17A Globemaster III approached Vnukovo International Airport in Moscow just after 10 a.m. local time (3 a.m. ET) after taking off from Riga, the capital of Latvia, two hours earlier.

The plane had departed from the U.S. Air Force’s Joint Base Andrews in Maryland on Sunday afternoon just before 5 p.m. ET., the data showed.