A senior security official in Ukraine said Thursday that investigators want to search the site where a Ukrainian plane crashed in Iran the day before for possible debris of a Russian missile, and said there had been a reference online to the existence of such debris.

Oleksiy Danilov, secretary of Ukraine’s Security Council, told Ukrainian media that officials had several working theories regarding the crash, including a missile strike.

“A strike by a missile, possibly a Tor missile system, is among the main (theories), as information has surfaced on the internet about elements of a missile being found near the site of the crash,” Danilov said. He did not elaborate on where he saw the information on the internet.

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An investigation team from Bellingcat, a journalism website that focuses on fact-finding, tweeted that it had seen an image purported to be Russian missile debris, but the angle of the picture meant it would be impossible to geolocate and prove it was at the crash site of the jetliner near Tehran.

The problem with this photo of the remains of an AA missile is it's been taken at an angle where it'll be next to impossible to geolocate, so unless another image appears which can be geolocated it won't be possible to verify it's in Iran.https://t.co/02rMEnhOYv — Bellingcat (@bellingcat) January 8, 2020

Ukrainian investigators who arrived in Iran earlier on Thursday currently await permission from Iranian authorities to examine the crash site and look for missile fragments, Danilov said.

The Tor is a Russian-made missile system. Russia delivered 29 Tor-M1s to Iran in 2007 as part of a $700 million contract signed in December 2005. Iran has displayed the missiles in military parades.

Iran did not immediately respond to the Ukrainian comments. However, Gen. Abolfazl Shekarchi, the spokesman of the Iranian armed forces, denied that a missile hit the airplane in comments reported Wednesday by the semiofficial Fars news agency. He dismissed the allegation as “psychological warfare” by foreign-based Iranian opposition groups.

Danilov also said other possible causes under consideration included a drone or another flying object crashing into the plane, a terrorist attack or an engine malfunction causing an explosion.

An Iranian report released Thursday said the plane’s crew never made a radio call for help and were trying to turn back for the airport when the plane went down.

The report suggested a sudden emergency struck the Boeing 737 operated by Ukrainian International Airlines early Wednesday morning, when it went down just moments after taking off from Imam Khomeini International Airport in Tehran.

Investigators from Iran’s Civil Aviation Organization offered no immediate explanation for the disaster, however.

Eyewitnesses, including the crew of another passing flight, described seeing the plane engulfed in flames before crashing, the report said. The crash caused a massive explosion when the plane hit the ground, likely because the aircraft had been fully loaded with fuel for the flight to Kyiv, Ukraine.

The report also confirmed that both of the so-called “black boxes” that contain data and cockpit communications from the plane had been recovered, though they had been damaged and some parts of their memory was lost. It also said that investigators have initially ruled out laser or electromagnetic interference as causing the crash.

According to aviation experts, only a handful of countries are capable of analyzing black boxes — notably Britain, France, Germany and the United States.

Ukrainian President Volodymyr Zelensky, meanwhile, said he planned to call Iranian President Hassan Rouhani about the crash and the investigation.

“Undoubtedly, the priority for Ukraine is to identify the causes of the plane crash,” Zelensky said. “We will surely find out the truth.”

The plane was carrying 167 passengers and nine crew members from several countries, including 82 Iranians, at least 63 Canadians and 11 Ukrainians, according to officials. The crash just before dawn scattered flaming debris and passengers’ belongings across a wide stretch of farmland.

It also came immediately after Iran launched a ballistic missile attack against Iraqi military bases housing US troops amid a confrontation with Washington over its killing of an Iranian Revolutionary Guard general in a drone strike last week.