By Catherine Nicholls, Hassan Tayir and Aruzhan Zeinulla, CNN

(CNN) — Azerbaijan held a day of mourning on Thursday for the dozens of victims of an airliner crash in Kazakhstan, as questions were being asked over the cause of the disaster.

The reasons why Azerbaijan Airlines flight J2-8243 came down are still unknown. Officials from Azerbaijan, Kazakhstan and Russia, three countries linked to the disaster, urged people not to speculate about the crash until investigations have concluded.

The plane was traveling from the Azerbaijani capital Baku to Grozny in the southern Russian republic of Chechnya before it made an emergency landing approximately 3 kilometers (1.8 miles) from Aktau, Kazakhstan, the carrier said on Wednesday.

Russian state media reported that the plane was rerouted due to heavy fog in Grozny.

Officials did not immediately explain why the plane had crossed the Caspian Sea, when Baku and Grozny are to its west and Aktau is to its east.

Some 29 survivors, including two children, were pulled from the wreckage, Kazakhstan’s Deputy Prime Minister Kanat Bozumbayev said.

Azerbaijani state news agency AZERTAC reported that 12 survivors will be transported back to Azerbaijan on Thursday, five of whom are in a “serious but stable condition.” These five people will be flown back by a special aircraft from the country’s Ministry of Emergency Situations, AZERTAC said.

Video and images of the plane after it crashed show perforations in its body that look similar to damage from shrapnel or debris. The cause of these holes has not been confirmed.

Azerbaijan Airlines initially told AZERTAC that the incident was caused by the aircraft colliding with a flock of birds, the outlet reported. Russia’s Federal Air Transport Agency Rosaviatsia also said that the plane crashed after colliding with birds.

However, Andriy Kovalenko, the head of Ukraine’s Center for Countering Disinformation, part of the National Security and Defense Council of Ukraine, disputed this, claiming on social media that the plane was “shot down by a Russian air defense system.”

The crash came shortly after Ukrainian drone strikes hit southern Russia. Drone activity has shut airports in the area in the past and the nearest Russian airport on the plane’s flight path was closed on Wednesday morning.

“Russia should have closed the airspace over Grozny but failed to do so,” Kovalenko said, speculating that authorities will try to cover up the real reason behind the crash, including the holes in the plane, as it would be “inconvenient” to blame Russia.

Kremlin spokesman Dmitry Peskov said on Thursday that it would be wrong to speculate about the cause of the plane crash before an investigation had been carried out, according to Russian state media RIA Novosti.

Maulen Ashimbayev, chairman of Kazakhstan’s senate, said Thursday that “the nature of these damages and the causes of the disaster are currently unknown.”

A commission has been set up to investigate the crash, involving representatives from Kazakhstan, Azerbaijan, and Russia, Ashimbayev said.

Kanat Bozumbayev, Kazakhstan’s deputy prime minister, said that “even the preliminary cause cannot be determined yet, as specialists are needed for that.”

“They will conduct the work, and then it will be clear,” Bozumbayev said Thursday.