An Azerbaijani airliner has crashed in Kazakhstan after being diverted, killing 38 of 67 people on board. Some experts alleged that the plane went down after being hit by Russian air defense systems.

Some things to know about the crash and the speculation about a possible cause:

How did the plane crash?

Azerbaijan Airlines’ Embraer 190 was en route from Azerbaijani capital Baku to the Russian city of Grozny in the North Caucasus on Wednesday when it was diverted for reasons that aren't fully clear yet. It crashed while making an attempt to land in Aktau, Kazakhstan, after flying east across the Caspian Sea.

The plane went down near the coast about 3 kilometers (around 2 miles) from Aktau. Cellphone footage circulating online appeared to show the aircraft making a steep descent before hitting the ground and exploding in a fireball.

Rescuers have rushed 29 people who survived the crash to hospitals.

How did Azerbaijan react?

Azerbaijan observed a nationwide day of mourning on Thursday. National flags were lowered across the country, traffic across the country stopped at noon, and signals were sounded from ships and trains.

Speaking at a news conference Wednesday, Azerbaijani President Ilham Aliyev said that the weather had forced the plane to change from its planned course.

What do they say about a possible cause?

Kazakhstani, Azerbaijani and Russian authorities say they are investigating the crash. Embraer told The Associated Press in a statement that the company is “ready to assist all relevant authorities.”

Russia’s civil aviation authority, Rosaviatsia, said that preliminary information indicated that the pilots diverted to Aktau after a bird strike led to an emergency on board.

Some experts alleged that the holes seen in the plane’s tail section pictured after the crash possibly indicate that it could have come under fire from Russian air defense systems fending off a Ukrainian drone attack.

Mark Zee of OPSGroup, which monitors the world’s airspace and airports for risks, said that the analysis of the fragments of the crashed plane indicate with a 90-99% probability that it was hit by a surface-to-air missile

Osprey Flight Solutions, an aviation security firm based in the United Kingdom, warned its clients that the “Azerbaijan Airlines flight was likely shot down by a Russian military air-defense system.”

Osprey CEO Andrew Nicholson said that the company had issued more than 200 alerts regarding drone attacks and air defense systems in Russia during the war.

In Azerbaijan, Caliber online newpspaper claimed that the airliner was fired upon by a Russian Pantsyr-S air defense system as it was approaching Grozny. It questioned why Russian authorities failed to close the airport despite the drone attack in the area on Wednesday, and why they didn’t allow the plane to land in Grozny or other Russian airports nearby after it was hit.

Asked Thursday about the claims that the plane had been fired upon by air defense assets, Kremlin spokesman Dmitry Peskov told reporters that “it would be wrong to make hypotheses before investigators make their verdict.”

Officials in Kazakhstan and Azerbaijan have similarly avoided comment on a possible cause of the crash, saying it will be up to investigators to determine it.