Inquiries into the A300 engine failure and run-off incident at Stockholm in January 2010 were held up for five months as a result of government sanctions against Iran.
Investigating authority SHK says the delay was due to the "politically determined" restrictions and the processing of applications needed to ensure compliance.
In particular, it says, a licence had to be granted before US support could be provided by General Electric, the engine manufacturer, or the National Transportation Safety Board.
Maintenance company Lufthansa Technik offered assistance to inspect the engine, but the potential transfer of technical data to Iranian entities meant it would only participate upon approval of GE's licence.
Stricter European Union sanctions against Iran also meant that examination of specific flight-data recorder parameters could not be carried out in Sweden, and took place in Iran.
SHK is recommending to the US FAA that processes to grant licences and waivers be improved so they "do not unnecessarily delay civil aviation safety investigations" involving US-built aircraft or parts.
While Lufthansa Technik could not "completely ascertain" the cause of the engine failure, it concluded that the failure originated from fatigue cracking of attachment lugs on the high-pressure turbine diffuser aft air seal.
But SHK sought a second opinion from Volvo Aero, which found that the primary cause of the failure was fatigue damage in a different part of the seal which had undergone repair. The seal then separated from the diffuser assembly.
SHK points out that, owing to various circumstances outside its control, the damaged engine was transported from Germany to Iran before any qualified examination had been carried out. It adds that Iran Air - on its "own initiative" and without SHK authorisation - disassembled the powerplant with no third-party oversight.