Engine problems sparked Heathrow emergency
British Airways pilots forced to make an emergency landing at Heathrow shut down an engine after it caught fire, an investigation team has said.
The Oslo-bound Airbus 319 returned soon after take-off on 24 May when black smoke was seen coming from an engine.
The US National Transport Safety Board, which is helping the UK's Air Accidents Investigation Branch said the engine cowls or coverings had fallen off.
The AAIB told the BBC its investigation into the incident continues.
The 75 passengers and crew on the flight were evacuated via emergency chutes after the plane landed safely at the west London airport.
The National Transport Safety Board (NTSB) was assisting the AAIB because the engines were American-made.
The board said a US accredited representative was looking into the manufacture of the engines as part of the investigation.
On its website the NTSB said: "An Airbus A319-131, registration number G-EUOE, during departure from London Heathrow International Airport, had the engine cowls from both International Aero Engines V2500 engines separate and fall on to the runway.
"The pilots reported that they shut down one engine, there was a fuel leak, and that they were returning."
They added: "The airplane landed, was shut down, and the passengers were evacuated via the emergency slides."
In 2008 the NTSB issued four safety recommendations to the US Federal Aviation Administration in relation to engine fan cowls coming off, including concerns over fastening of the cowl latches after maintenance.
There were three incidents in 2008 and one in 2007, three of which involved the Airbus A320 "family" of aircraft, which includes the A319.
The BBC's Transport Correspondent Richard Westcott said: "What we do now know from the NTSB is that the plane lost both engine cowlings during take-off.
"The fact that they both broke at about the same time points towards a problem with the latches that hold them in place.
"That could mean a mechanical fault or it could mean the engineering team hasn't locked and checked them properly after maintenance."
He added: "We don't yet know for certain what has caused this problem but I have spoken to a number of experts and so far, failing to lock the engine "bonnet" properly is looking like the most likely explanation."