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.”

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.

Russian President Vladimir Putin apologized for the “tragic incident,” which left 38 people dead, though he stopped short of taking responsibility.

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

Azerbaijan’s leader on Monday accused Moscow of carrying out a “cover up” over a passenger jet crash last month that claimed 38 lives, as relations sour between the two neighbors.