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.”

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.

Russian aviation authorities said the Azerbaijan Airlines plane had hit a flock of birds. But some experts cast doubt on that account, pointing to footage showing apparent holes in the fuselage.

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

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.

Azerbaijan Airlines flight J2-8243 caught fire as it tried to make an emergency landing in Aktau, southwestern Kazakhstan, on Wednesday. It was traveling from the Azeri capital Baku to Grozny, the capital of the Russian republic of Chechnya.

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.