After an Azerbaijan Airlines jet crashed in Kazakhstan, speculation mounted about why the plane went down and whether possible Russian air defense action may have played a role.

Russians who reviewed the transcript of the crew's conversations from the Baku-Grozny flight themselves admit that the air crash was caused by their air defense systems (ADS), according to Andriy Kovalenko,

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.

Aviation experts say that Russian air defense fire was likely responsible for the Azerbaijani plane crash the day before that killed 38 people and left all 29 survivors injured.

"No one claims that it was done on purpose," an Azerbaijan official told Reuters. They added their government "expects the Russian side to confess."

Russian air defenses, rather than a bird strike, likely caused the downing of an Azerbaijan Airlines flight, killing 38 people.

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.

A passenger plane flying from Azerbaijan to Russia with 67 people on board crashed in Kazakhstan Wednesday, authorities said.

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.