After an Azerbaijan Airlines jet crashed in Kazakhstan, speculation mounted about why the plane went down and whether possible Russian air defense action may have played a role.

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.

The cause of the Azerbaijan Airlines tragedy is under investigation, but the company is crediting “the crew's valiant dedication to their duties until the last moment”