Early indications suggest a Russian anti-aircraft system may have downed the passenger jet that crashed in Kazakhstan on Wednesday, a US official told CNN as Azerbaijan held a day of mourning for the dozens of victims of the disaster.

Azerbaijan Airlines' Embraer 190 crashed near Aktau in Kazakhstan, killing 38 people. Experts suspect Russian air defense fire as the cause. Azerbaijan, Kazakhstan, and Russia are investigating, while theories suggest that the plane was hit by a surface-to-air missile,