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.”

Russian President Vladimir Putin apologized for the plane crash in Russian air space on Christmas Day that left 38 people dead in a statement that said Russia was targeting Ukrainian drones over the plane’s planned destination at the time, but stopped short of taking responsibility.

Azerbaijani President Ilham Aliyev on Sunday demanded that Russia admit that it fired at an Azerbaijan Airlines passenger jet before it crashed, accusing Moscow of initially trying to cover up the cause of the fatal disaster.

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.