The exact cause of the crash is still under investigation, but a U.S. official has preliminarily suggested that a Russian anti-aircraft system might have shot down the plane.

Russian President Vladimir Putin on Saturday apologized to his Azerbaijani counterpart for what he called a “tragic incident” following the crash of an Azerbaijani airliner in Kazakhstan that killed 38 people.

Russian President Vladimir Putin said Saturday that Russia’s air defense systems were active when an Azerbaijan Airlines flight attempted to land in Grozny, Chechnya on Wednesday.