Portuguese investigators suspect that the fuel injector system was the source of damage to a General Electric GE90 engine on a TAAG Angola Boeing 777-200 shortly after departure from Lisbon last month.
The aircraft shed parts on the region of Almada, as it climbed to 5,000ft on 6 December, damaging cars and buildings.
Its right-hand GE90 was shut down by the crew and the 777 returned to Lisbon where it landed without further incident.
Portugal's investigation agency, the GPIAA, says the high-pressure turbine area was in good shape but there was "substantial damage" to the last stage of the low-pressure turbine, with deformation and material loss in the turbine blades and guide vanes.
Impact marks were also found on the aircraft's fuselage and horizontal stabiliser leading edge.
Technical logs from the aircraft turned up previous flight crew remarks that the right-hand engine's exhaust gas temperatures had been running around 35-45° high.
GPIAA states that the evidence of an "excessive temperature build-up" in the turbine area and the failure of low-pressure turbine guide vanes could point to "abnormal behaviour" by one or more fuel injectors.
TAAG ordered an examination of the engines on its 777 fleet - comprising three aircraft - before reportedly suspending operations with the type after a second suffered an engine problem on take-off, on 23 December.
Last year TAAG signed an agreement with GE covering maintenance support for the engines on its three 777s. But GE Aviation points out that the engine in this event has not had its first shop visit, and its pact with the airline does not cover day-to-day line maintenance.