The two black boxes, which record cockpit conversations and flight data, have been recovered following the crash of a Ukrainian passenger aircraft shortly after taking off in Tehran.
There has been speculation that the three-year-old plane, in which all 176 people on board were killed including three Britons, could have been downed by a missile, similar to what happened to Malaysia Airlines flight MH17 over Ukraine in July 2014.
As Ukrainian envoys work at the crash site outside Tehran, what could be some of the possible causes of Wednesday’s disaster?
Tony Cable is a former senior inspector at the UK Air Accidents Investigation Branch (AAIB) and offers some potential theories.
He told Sky News: “I’ve seen a few photos of the crash site. The wreckage shows a high degree of break up which indicates that the plane crashed vertically at speed into the ground. There seems to be a lot of relatively small pieces of debris.”
Engine bay fire
The Ukrainian embassy in the Iranian capital originally pointed to engine failure as the cause of the Boeing 737-800 disaster but dropped that reference in later comments.
An Iranian transport official has been quoted as saying it appeared a fire struck one of its engines and then the pilot lost control.
Mr Cable said if there was a fire in one engine then under the normal protocol the fuel supply would be turned off and the plane would be able to be flown with just the one good engine.
It appears the plane crashed vertically and if it was simply an engine fire the pilot would be able to shut down the affected engine and remain in control of the aircraft.
The crash was the first fatal incident involving Ukraine International Airlines (UIA) and the plane was flown by experienced pilots, company officials said.
Iranian officials said they suspected a mechanical issue brought down the aircraft.
Mr Cable said many mechanical issues are possible. However, the Boeing 737 record is relatively good, excluding the Boeing 737 MAX.
Flight control problem
Mr Cable said: “Flight controls can go wrong but it is pretty unlikely. It happened with the Boeing 737 MAX aircraft with two tragedies in Indonesia and Ethiopia in recent years.”
Although the Boeing aircraft involved in Wednesday’s crash was from the same family as the MAX, it was different in that it operates with a different software system to the one implicated in the MAX crashes, he said.
The plane was shot down
The passenger jet came down in farmland outside Tehran hours after Iran launched rocket attacks against two bases in neighbouring Iraq housing US troops.
Mr Cable said: “There would be some pretty obvious characteristics if this was the case. There would be shrapnel damage to parts of the wreckage.
“Investigators would collect the wreckage on site and then lay it out in the form of the aircraft.”
He went on: “In the case of MH17 – it was pretty obvious for investigators that it had been hit by a missile once they saw the wreckage.”
Non-contained engine failure
Mr Cable said this is where high energy debris from an internal failure of the engine penetrates the engine casing.
This debris can damage the airframe and systems, such as fuel lines, hydraulic lines, electrical cables in the wing.
Massive electrical fire
An uncontrolled electrical fire would be rare, he said.
A bomb detonated on board
Mr Cable said: “A bomb could cause loss of control and a high vertical speed crash. Investigators would be looking for wreckage that is separated from the main wreckage.
“So in a bombing, small pieces from the initial blast would be found somewhere else from the main structure.”
Meanwhile, the head of Iran’s civil aviation organisation has been quoted as saying it was not clear which country Iran would send the black boxes to for data analysis, but it would not give them to Boeing.