Questions were being asked Thursday about the cause of an airliner crash in Kazakhstan the day before, as Azerbaijan held a day of mourning for the dozens of victims of the disaster. The reasons that Azerbaijan Airlines flight J2-8243 came down are still unknown.

The flight was en route from the Azerbaijani capital of Baku to Grozny, in Russia’s south, when it diverted and attempted an emergency landing in Kazakhstan.

NATO called for a formal investigation into the Azerbaijan Airlines plane crash that killed 38 people in Kazakhstan on Thursday. “Our thoughts and prayers are with the families and victims of