Why was US Navy transporting 220,000 barrels of jet fuel in stricken tanker?
The Stena Immaculate, a US-flagged oil tanker, suffered a ruptured cargo tank and a gaping hole in the hull after she was struck by the Portuguese-registered container ship Solong in the North Sea.
Three days later, the fire on the Stena Immaculate, which was carrying 220,000 barrels of military jet fuel, is finally out and "pockets of fire" continue to burn on the Solong, as investigations continue into the cause of the crash.
The question now is what went so badly wrong – and why?
The Marine Accident Investigation Branch is investigating, alongside US and Portuguese authorities, but any findings may not be made public for some time.
So, what do we know so far?
Tanker headed for UK loaded with military jet fuel
The Stena Immaculate was chartered by the US Navy's Military Sealift Command and was carrying around 220,000 barrels of Jet A-1 fuel – this is a normal procedure.
The Sealift Command operates around 125 civilian-crewed vessels to transport fuel around the world.
The fuel on board the Stena Immaculate is a type of kerosene intended for specific use by the US military.
It is compatible with most jet aircraft, helicopter turbine engines, turboprops and compression-ignition piston engines.
The fuel was stored in 16 segregated cargo tanks aboard the Stena Immaculate.
Crowley, the maritime company managing the ship, says at least one tank ruptured when she was struck.
The company is still unsure exactly how much fuel was lost.
According to the US Defence Logistics Agency, which manages the US military's global supply chain, the tanker was heading from Greece towards the port of Killingholme, Lincolnshire, on the southern side of the Humber estuary.
She was expected to arrive there on 9 March.
Marine traffic data indicates the tanker was anchored at the time of the collision.
Built in 2107, she is 183 metres long and 32 metres wide and should have been clearly visible to passing ships.
The cargo ship on a collision course
The Solong, a cargo ship, was sailing under the Portuguese flag from the Scottish port of Grangemouth to Rotterdam when she hit the Stena Immaculate.
At 140 metres long and 22 metres wide, she is slightly smaller than the tanker.
German operator Ernst Russ confirmed the ship was not carrying containers of sodium cyanide, as had been previously reported.
But she also suffered extensive damage from the intense fire on board.
How could this collision happen?
UK Transport Minister Mike Kane says there is no evidence of foul play.
But maritime experts are puzzled as to why Solong did not respond to the ship's modern alarm systems or visually identify the Stena Immaculate up ahead.
Witnesses reported fog at the time and maritime safety records seem to indicate that the Solong may have failed recent tests, but this still does not exactly explain the collision.
Investigators hope to learn more when the fires on both ships are fully extinguished, so experts can safely board the damaged ships to make a full assessment.
US and Portuguese officials are expected to work alongside Britain's Marine Accident Investigation Branch to determine the cause of the crash.
What are the consequences for the environment?
Despite initial fears, the coastguard has assured the public there is no sign of pollution at the crash site as it is thought the resulting fires burned much of the spilled oil, limiting the environmental impact.
But jet fuel is highly toxic to marine life and environmental groups are still concerned about the implications for wildlife.
The good news is that neither ship is thought likely to sink.
Although the Stena Immaculate suffered catastrophic damage with a huge hole in her hull, her cargo was carried in multiple tanks, making the ship more stable and remain sealed if water enter one compartment.
A large clean-up operation, however, lies ahead.
Why does the US military transport fuel to the UK?
The US Air Force has 13 bases on British soil.
These are RAF Lakenheath, RAF Croughton, RAF Welford, RAF Fairford, RAF Feltwell, RAF Upwood, RAF Barford St John, RAF Blenheim Crescent, RAF Fylingdales, RAF Menwith Hill, RAF Mildenhall, RAF Alconbury and RAF Molesworth.
The US Air Force operates numerous aircraft, including the F-35A Lightning II, F-16 Fighting Falcon and B-52H Stratofortress.
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