Azerbaijan Airlines’ Embraer 190 was en route from Baku to the Russian city of Grozny when it was diverted for reasons yet unclear and crashed while making an attempt to land in Aktau in Kazakhstan.

The Azerbaijan Airlines plane flew hundreds of miles off its scheduled route from Baku to Grozny, and crashed on the eastern shore of the Caspian Sea. Russia's aviation watchdog said a bird strike might have been the cause.

The flight was en route from the Azerbaijani capital of Baku to Grozny, in Russia’s south, when it diverted and attempted an emergency landing in Kazakhstan.

An Azerbaijan Airlines flight to the southern Russian town of Mineralnie Vody turned back to Baku on Friday after being told a chunk of airspace in southern Russia had been closed, Russia's TASS news agency reported.

Early indications suggest a Russian anti-aircraft system may have downed the passenger jet that crashed in Kazakhstan on Wednesday, a US official told CNN as Azerbaijan held a day of mourning for the dozens of victims of the disaster.

There was growing speculation that an Azerbaijani airliner was shot down by Russian air defense system. Russia had initially said it believed the plane hit a flock of birds, but a U.S. official says it may have been struck by Russian air defenses.