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.

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.

The Azerbaijan Airlines flight from Baku to Grozny in Russia was carrying 67 people when it crashed near Aktau early Wednesday. Kazakh officials said 29 people survived.