Questions were being asked Thursday about the cause of an airliner crash in Kazakhstan the day before, as Azerbaijan held a day of mourning for the dozens of victims of the disaster. The reasons that Azerbaijan Airlines flight J2-8243 came down are still unknown.

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.

Dozens of people were killed on Wednesday after an Azerbaijan Airlines flight crashed near the Kazakhstani city of Aktau, according to officials.