'Ice in fuel' caused BA 777 crash

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   The key phrase in this article is 'may have', this is only an interim report from the AAIB:

The BA plane that crashed at Heathrow in January was probably brought down by ice in its fuel system, an accident report says.
The pilots of the Boeing 777 managed to land safely, and 136 passengers and 16 crew escaped without serious injury.
The Air Accident Investigation Branch believes the flow of fuel
dropped causing the engines to lose power less than a minute before
touchdown.
But investigators say they still do not know how the ice could have formed.
Water is naturally present in aviation fuel and the
investigators believe there may have been as much as 5 litres within
this aircraft's fuel load.

But the report says levels of water found in fuel recovered after the 17 January crash were very low for a Boeing 777

Siberian temperatures
They dismissed the suggestion that the fuel itself froze or became "waxy" as icing occurred.
The report says the plane flew through unusually cold air over Siberia while en route from Bejing to Heathrow. 
The fuel temperature fell to -34C (-29F). But jet fuel should not
freeze until it is at less than -57C (-70F), and the report says the
temperatures involved were not "unique".
The investigation into the crash of flight BA38 continues with
testing at Rolls Royce in Derby, and Seattle in the US, the home of
Boeing.
Water in aviation fuel can be dissolved at the molecular level,
or simply float as free water, suspended in the fuel. As the fuel gets
colder tiny droplets can form and freeze.
The mystery facing investigators is why this might have happened on an apparently fully-functioning aircraft.
Water in the fuel is controlled by draining it regularly out of
the fuel tanks - and on the Boeing 777 a so-called "scavange system"
pumps it out.
Ice can form when the fuel temperature drops to around -1C
(30.2F) to -3C (26.6F) Generally the ice crystals simply float and
drift in the fuel without causing harm. 
Only when the temperature falls further does the ice stick together.
Within the fuel system a heat exchanger is used to increase the fuel
temperature, but it is possible the blockage might have occurred before
this point.
The investigation team have built a test rig and introduced pre-prepared ice into the fuel system to see if it would clog up.
But the amounts they had to put in to make this happen were far greater than is normal.
Despite that, the scenarios being considered by the AAIB are
based on the idea that the ice formed gradually in the system and was
released as the plane prepared for landing.
But the report makes three safety recommendations:
that the US Federal Aviation Administration and the European
Aviation Safety Agency introduce interim measures to reduce the risk of
ice forming on the Boeing 777 powered by Trent 800 engines 
the agencies should consider the implications for other aircraft types 
review the requirements for new engines 
This accident remains an enormous challenge for the investigation team. But their report stresses the rarity of this crash.
'Unique' situation
It said: "The accident flight was unique in that this has been
the only recorded case of a restricted fuel flow affecting the engine
performance to the extent of causing HP pump cavitation" - the damage
found within the pumps that alerted the investigators to the loss of
fuel pressure.
The report goes on: "This is the first such event in 6.5
million flight hours and places the probability of the failure as being
'remote'."
Flight BA 38 from Beijing was extensively damaged when it
touched down on 17 January, landing short of the runway but staying
largely intact.
One person suffered a broken leg and others received minor injuries during the emergency evacuation.