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.

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.

Dramatic video shows an Azerbaijan Airlines plane hitting the ground and bursting into flames near the city of Aktau in southwestern Kazakhstan. Dozens of passengers were killed, but video from the crash site also shows some of the survivors were able to walk away from the wreckage.