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.

Thursday is an official day of mourning in Azerbaijan as an investigation is underway to determine what caused an airplane carrying 67 passengers to crash on Christmas Day in Kazakhstan, killing 38 and leaving at least 29 survivors.

We will never forget the beloved people we lost in the crash of the Embraer 190 aircraft,” said Azerbaijan Airlines in a statement posted online on Boxing Day. The pro-government AnewZ news station cited Azerbaijan officials as saying shrapnel from a Russian missile was responsible.