Thursday is an official day of mourning in Azerbaijan as an investigation is underway to determine what caused an airplane carrying 67 passengers to crash on Christmas Day in Kazakhstan, killing 38 and leaving at least 29 survivors.

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.

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.