Azerbaijan President Ilham Aliyev said Russia is behind the Christmas Day crash of an Azerbaijan Airlines plane in Kazakhstan, which killed 38 of the 67 people aboard. The motives of the incident remain unclear.

The leader of Azerbaijan criticized the Russian response to the crash of a passenger jet that Azerbaijani officials said had most likely been hit by Russian air defenses.

Two U.S. military officials told NBC News that they had intelligence indicating that Russians may have misidentified the plane and shot it down.

Several passenger plane crashes have occurred worldwide this year, including a Wednesday morning Azerbaijan Airlines wreck that has left dozens feared dead.

In the crash’s aftermath, Azerbaijan has unleashed rare and stinging criticism of Russia, with the country’s president saying Moscow’s response has caused “surprise, regret and rightful indignation.”