Engine cowls, or coverings, on both the US-made engines of the Oslo-bound Airbus A319 came off and fell on the runway as the plane took off from Heathrow on May 24, the US National Transport Safety Board (NTSB) team concluded.
The pilots reported that they shut down one engine and there was a fuel leak which meant they had to return.
"The pilots subsequently reported that one engine was shut down and the other engine was on fire," the NTSB said.
Eye witnesses on the ground had reported seeing smoke billowing from the stricken plane as it flew over London before landing.
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All 75 passengers and crew were evacuated via emergency chutes, resulting in minor injuries sustained by three people on board.
The incident closed both runways at one of the world's busiest airports for a while and although they re-opened soon after, it caused a number of flight cancellations and delays.
"The pilots reported that they shut down one engine, there was a fuel leak, and that they were returning. The airplane landed, was shut down, and the passengers were evacuated via the emergency slides."
BA cancelled almost 200 flights in the wake of the incident but the airline has insisted it is not liable for further compensation to anyone travelling to Europe under European Union consumer legislation.