A day after an Azerbaijani airliner’s crashed in Kazakhstan, aviation experts say the plane most likely went down after being hit by Russian air defense systems.

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.

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.