Doubts have been raised over a US airline's claim that its diversion of a transatlantic flight from London was due to odour from a spilled cleaning product.
The incident on board American Airlines Flight 729 saw two crew and one passenger taken to hospital.
The BBC has seen documents showing that the Airbus A330 aircraft was leaking oil before the flight from Heathrow to Philadelphia.
Sources told the corporation it is likely the leak led to toxic fumes entering the cabin.
This has been denied by American Airlines.
Flight 729 took off from Heathrow bound for the US on October 21, but was diverted to Dublin.
One of the pilots on board said two cabin crew had "lost consciousness", according to audio appearing to be from the radio conversation between the flight crew and an air traffic controller in the Irish capital.
A statement issued by American Airlines at the time said the incident was "due to an odour caused by a spilled cleaning solution in the galley".
A source from the airline told the BBC it was "inconceivable" that a cleaning product approved for use on a plane would cause people to pass out.
The BBC reported that an oil leak in the plane's auxiliary power unit (APU) was discovered days before the incident, and a noxious odour resulting in "eye and throat irritation" was recorded on the same aircraft shortly after the diverted flight.
American Airlines said in a statement that the APU was out of service at the time of the flight, but the BBC cited an Airbus document entitled "APU bleed air oil contamination" which warns that an oil smell or smoke in the cabin can be caused by "APU oil contamination", even when the device is not running.
The airline said the plane passed a maintenance check in Dublin on October 22, but after boarding on that day "some of our crew members believed they detected the odour once again" so passengers were re-booked on alternative flights.
No further odours were reported while the plane was flown back to Philadelphia with no passengers on board, the airline said.
The statement went on: "Cabin odours are a priority for American's leadership team at the highest level of the organisation.
"We will continue to apply our industry leading techniques and monitor all scientific, engineering and medical resources available.
"It cannot be emphasised enough that the health and welfare of our crews and customers continues to be our top priority.
"However, in the case of this aircraft and the diversion to Dublin, there is no connection to the APU or bleed air from the APU."