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.

U.S. officials say Russia could be behind a passenger plane crash in Kazakhstan that left dozens dead on Christmas Day. U.S. officials say it looks like the plane was hit by a Russian air defense system,