Russian President Vladimir Putin said Saturday that Russia’s air defense systems were active when an Azerbaijan Airlines flight attempted to land in Grozny, Chechnya on Wednesday.

The exact cause of the crash is still under investigation, but a U.S. official has preliminarily suggested that a Russian anti-aircraft system might have shot down the plane.

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.

Four sources with knowledge of the preliminary findings of Azerbaijan's investigation told Reuters that Russian air defences mistakenly shot it down.

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

President Ilham Aliyev, an ally of president Vladimir Putin, has accused the Russian regime of trying to cover up a plane crash which killed 38 people on Christmas Day

Russian President Vladimir Putin apologized for the deadly Azerbaijan Airlines Flight 8243 crash on Christmas but did not take responsibility for causing it as an investigation continues into the matter.

Ilham Aliyev, the President of Azerbaijan, has demanded 'justice' and 'punishment' for those responsible for the Christmas Day plane crash that killed 38 people

Azerbaijan's president has confronted Putin over the devastating plane crash that killed 38 people on Christmas Day.

The head of a Baku-affiliated think tank called Moscow’s response to the plane crash a display of “imperial arrogance,” with the Azerbaijani president lashing out at Moscow’s inadequate responses.

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.”