A suspected stowaway was found unconscious on a flight from Johannesburg after the plane landed in London and a second man may have died falling out of the same flight, British police said Friday.
The man found alive on Thursday is believed to be 24 years old and police said that he had been hiding in the undercarriage of the British Airways plane at the end of a journey that usually lasts 11 hours.
"His condition is now described as critical," a police statement said.
Police had found the body of the dead man on the roof of an office under the flight path to London Heathrow Airport and were trying to work out if he could have fallen out of the plane.
A police spokesman told AFP that the possibility was "one of the lines of inquiry."
The police were called about the suspected stowaway on the 12,875-kilometre (8,000-mile) flight from South Africa at 8:28 am (0728 GMT) and about the body at 9:35 am (0835 GMT).
The flight takes 11 hours and outside temperatures during the journey would fall as low as minus 60 degrees Celsius (minus 76 Fahrenheit).
"We are working with the Metropolitan Police and the authorities in Johannesburg to establish the facts surrounding this very rare case," a British Airways spokeswoman said.
The body was found on the roof of the offices of notonthehighstreet.com, an online retailer, in Richmond, a wealthy part of southwest London.
"Officers and the London ambulance service attended and found the body of a male on the roof of the premises," the company said in a statement.
"The death is currently being treated as unexplained but early indications are that the body may be that of an airline stowaway," the statement said.
Hady Khoshkbary, who runs a printing shop next door, said: "We were very lucky that the body did not drop on the road.
"Already this is a tragedy but that would have been horrific to see this incident," he added.
There have been several cases of stowaways being found dead clinging to the landing gear of planes.
In 2012, a man from Mozambique fell from the undercarriage of a Heathrow-bound flight from Angola onto a street under the flight path near Richmond.
An inquest found that he may have survived freezing temperatures for most of the flight but was "dead or nearly dead" by the time he hit the ground.
"It's very shocking when it's so close to you," said Reverend Neil Summers from the St John the Divine Church opposite where the body was found on Thursday.
"In one sense it's not totally surprising as it's happened before," he said.
In April, an Indonesian stowaway survived an hour-long flight from Sumatra to Jakarta hidden in the undercarriage of the plane.