Seven theories about what caused Iran plane crash
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 ...