Even as a probe was starting of the crash of an Azerbaijan Airlines airliner in Kazakhstan, speculation was mounting about why the plane went down.

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.

A U.S. official tells NBC News there are early indications that the passenger plane that crashed in Kazakhstan may have been hit by a Russian anti-aircraft system. The official said if it turns out to be the case,

HAUNTING footage shows passengers on board the doomed Azerbaijan Airlines flight just moments before it crashed – killing 38 people. Dozens of passengers, including children, miraculously

Azerbaijan Airlines flight J2-8243 had flown hundreds of miles off its scheduled route from Azerbaijan to Russia to crash on the opposite shore of the Caspian Sea in Kazakhstan. A Kazakh official said 38 people had been killed,

An Azerbaijan Airlines passenger aircraft crashed near Kazakhstan's Aktau Airport close to the Caspian Sea on Wednesday morning.

Russia’s state aviation authority said the Azerbaijan Airlines plane had been trying to make an emergency landing. The Kazakh authorities said that at least 29 people survived.

The Azerbaijan Airlines flight from Baku to Grozny in Russia was carrying 67 people when it crashed near Aktau early Wednesday. Kazakh officials said 29 people survived.

Azerbaijani President Ilham Aliyev said at a news conference that it was too soon to speculate on the reasons behind the crash, pointing to bad weather as the reason the plane diverted from its original course. "This is a great tragedy that has become a tremendous sorrow for the Azerbaijani people," he said.

Footage shows the moment an Azerbaijan Airlines flight crashes in Kazakhstan on Wednesday. Sources say it was downed by the Russian air defence system. Video credit: Social media

A Russian surface-to-air missile caused the crash of the Azerbaijan Airlines flight near Aktau, Kazakhstan, Azerbaijani government sources reportedly confirmed to Euronews on Dec. 26.