With 260 casualties and only one surviving passenger, the Air India 171 crash is one of the deadliest aviation incidents in recent history — and so far it’s proving to be one of the most frustratingly opaque.

FAA Administrator Bryan Bedford said Thursday that the June crash of an Air India flight that killed 260 people does not appear to have been caused by a mechanical problem with the aircraft's fuel control switches, which have emerged as a major focus in the Indian government's investigation into the disaster.