Iranian investigators have established a working group to examine the possibility of unlawful action against the Ukraine International Airlines Boeing 737-800 which crashed outside Tehran on 8 January.

The Civil Aviation Organisation says it has set up several groups to look into various aspects of the crash, in line with ICAO standards. But it also says that, given the evidence of a fire on the aircraft, an additional group is being convened to investigate any unlawful action.

CAO chief Ali Abedzadeh insists that a missile strike on the 737 would have been “scientifically impossible”, pointing out that other domestic and international aircraft were operating in Iranian airspace at the same time.

He adds that there is “complete co-ordination” between military and civil sectors in Iran.

While there is no verified evidence of a missile attack on the aircraft, the circumstances of the crash make the possibility of hostile action – albeit unintentional – difficult to dismiss so early into the investigation, particularly with the apparent outbreak of fire within moments of departure.

Most notable is the timing of the event, coinciding with heightened military activity as Iranian defence forces launched an overnight missile attack on US targets in Iraq.

Ukraine International Airlines has already disclosed that the crew was highly experienced. The aircraft was delivered new in 2016 and was fitted with CFM International CFM56 engines – powerplants with a high level of reliability.

There is no immediate indication of a powerplant problem but, even with a serious uncontained failure, the rapid loss of control and destruction of the aircraft – given that pilots are routinely trained to deal with engine failure after take-off – would be highly unusual.

The absence of a distress call or other transmission could point to the crew’s prioritising the handling of the aircraft. But the sudden cessation of publicly-available data transmissions from the aircraft, as it climbed to its assigned altitude, could also indirectly indicate a situation that overwhelmed its systems.

Most of the occupants of the aircraft held Iranian passports, according to the inquiry, and Ukrainian-Iranian relations have recently appeared sound – Iranian president Hassan Rouhani mentioned strong and friendly relations between the two countries in an independence day message to his Ukrainian counterpart in August.

This would suggest that a deliberate attack on the 737 is unlikely, but does not rule out an inadvertent mishap.

CAO chief Abedzadeh denies that Iranian authorities are resisting transferring the flight recorders of the aircraft to US investigators for analysis, but points out that the inquiry has various options for seeking assistance to download the recorder information – including sending the devices to France or another third country.

Ukraine’s president, Volodymyr Zelensky, has ordered a routine criminal investigation into the crash, through the country’s prosecutor general.

He has also insisted that all civil aircraft in the Ukrainian fleet will undergo an airworthiness review, to verify their technical condition.

“Regardless of the conclusions regarding the causes of the Iranian catastrophe, the airworthiness of the entire civilian fleet will be tested,” says Zelensky.

“Undoubtedly, the priority for Ukraine is to identify the causes of the aircraft crash,” he adds. “We will find out the truth. A thorough and independent investigation will be conducted in accordance with international law.”

Zelensky says a large team of Ukrainian representatives has been sent to Iran to participate in the probe, in particular the downloading and analysis of the aircraft’s flight recorders.

Ukrainian airlines are being banned from flying in Iranian and Iraqi airspace until all the reasons for the crash are established, he adds.