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.

Russian aviation authorities said the Azerbaijan Airlines plane had hit a flock of birds. But some experts cast doubt on that account, pointing to footage showing apparent holes in the fuselage.