Azerbaijan Airlines’ Embraer 190 was en route from Azerbaijani capital of Baku to the Russian city of Grozny in the North Caucasus on Wednesday when it was diverted for reasons that aren’t fully clear yet. It crashed while making an attempt to land in Aktau, Kazakhstan, after flying east across the Caspian Sea.

Mobile phone footage circulating online appeared to show the Azerbaijani Airlines plane making a steep descent before slamming into the ground in a fireball in western Kazakhstan.

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.