As the official investigation into the crash of the Azerbaijan Airlines plane began, the scenario now being considered is whether it was shot down by anti-aircraft fire.

Azerbaijani and US officials told Reuters on Thursday that based on preliminary findings, the plane appeared to have been hit by Russian fire.

Some experts say the holes seen in the plane’s tail section suggest it could have come under fire from Russian air defense systems, such as those used to repel Ukrainian drone attacks.

Ukrainian drones have previously attacked Grozny, the provincial capital of the Russian republic of Chechnya, and other regions in the country’s North Caucasus. Some Russian media outlets claimed there was another drone attack in Chechnya on Wednesday, although this has not been officially confirmed.

Yan Matveyev, an independent Russian military expert, noted that images of the crashed aircraft’s tail reveal damage consistent with shrapnel from a small surface-to-air missile, such as the Pantsyr-S1 air defense system.

“It appears that the tail section of the plane was damaged by some missile fragments,” he said.

Osprey Flight Solutions, a UK-based aviation security company, warned its clients that “the Azerbaijan Airlines flight was likely shot down by a Russian military air defense system.” Osprey provides analysis of carriers still flying to Russia after Western airlines suspended flights during the Ukraine war.

Osprey chief executive Andrew Nicholson said the company had issued more than 200 alerts about drone attacks and air defense systems in Russia during the war.

“This incident is a stark reminder of why we do what we do,” Nicholson wrote online. “It is heartbreaking to know that despite our best efforts, lives were lost in a way that could have been avoided.”

Russian military expert Yuri Podoliaka told AFP that the holes seen in the plane’s wreckage were similar to those left by an “anti-aircraft missile system”. “Everything points to this,” he wrote.

And Gérard Legofret, a former expert with France’s air accident investigation agency, also said there appeared to be “many gunshot wounds” in the wreckage, according to AFP.

In fact, he likened the markings to those on Malaysia Airlines Flight MH17, a passenger jet that was shot down by a surface-to-air missile by Russian-backed rebels over eastern Ukraine in 2014.

But when asked about claims that the plane had been hit by Russian fire, Kremlin spokesman Dmitry Peskov told reporters: “It would be wrong to make assumptions before the investigators make their verdict.”