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.

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.

Even as a probe was starting of the crash of an Azerbaijan Airlines airliner in Kazakhstan, speculation was mounting about why the plane went down.