The Kremlin on Thursday warned against speculating what may have caused an Azerbaijan Airlines flight to crash, killing 38 of the 67 on board, after an aviation expert said evidence indicated that a Russian anti-missile battery may have brought down the passenger plane.

Azerbaijan held a day of mourning on Thursday for the dozens of victims of an airliner crash in Kazakhstan, as questions were being asked over the cause of the disaster.

The flight was en route from the Azerbaijani capital of Baku to Grozny, in Russia’s south, when it diverted and attempted an emergency landing in Kazakhstan.