A US nuclear submarine came close to running aground, following an incident in which two crewmen died as it tried to leave one of Britain's main naval bases, a report has revealed.
Two US sailors, Chief Petty Officer Thomas Higgins and Petty Officer Michael Holtz, died as the attack submarine USS Minneapolis-St Paul tried to exit Plymouth Sound after a visit to Devonport Naval base in December 2006.
Three others were swept into the rough seas before being rescued by nearby boats.
The 2007 Royal Navy report into the incident, released this week under the Freedom of Information Act, said that the 110m-long, 6,000 ton vessel, "came within less than her own length" of hitting rocks and becoming stuck with "catastrophic consequences" as she turned to get back into protected waters.
The report said that the incident, which came as the Devonport harbour pilot was trying to disembark the submarine, was largely the fault of the vessel's commanding officer.
But it also criticised a lax safety culture at the naval base, the largest in Western Europe, including failing to heed warnings after a similar but non-fatal accident involving British submarine HMS Sovereign the previous February.
"This was a severe incident with multiple loss of life. There was a very real possibility of the boat grounding in very rough seas and on an ebb tide 500 yards south of Plymouth breakwater," the report said.
"In addition the crew's mess hatch remained open in these conditions allowing a considerable volume of water into the submarine.
"Tragic as the loss of the lives of Holtz and Higgins was, the outcome could have been so much more catastrophic and thus must be regarded as at the less serious end of the potential spectrum of consequences."
The report describes how the harbour pilot tried to leave the submarine too late as, at nine knots, it started to pass west of the Plymouth Sound breakwater and faced the open sea, which had a swell of up to four metres in height and wind speeds of up to 35 knots.
The report said the crew of a nearby vessel became alarmed at the submarine's speed as it approached the open water.
As the harbour pilot tried to leave through the mess hatch, the commanding officer swung the sub round into the waves, exposing the men on the desk "casing", who were there to help the harbour pilot to the waiting boat, to the full force of the sea.
The two men who died were moored to the submarine with safety lines and were repeatedly dashed "like rag dolls" against the hull by the force of the waves.
As the submarine turned to return to the safety of protected water, she came close to the rocks of the Panther Shoal, which would have been enough in the conditions, the report said, to ground her.
The mess was already around 18 inches deep in seawater which had come in through the open hatch.
The report said "human failure lay at the heart of this incident" and although saying responsibility for the incident lay with the submarine commander, it also recommended changes to the safety culture within Devonport to make it easier and quicker for concerns to be reported and acted upon.
The harbour pilot was also later dismissed, a decision upheld by the Civil Service Appeal Board.
An MoD spokesman said: "Naval Base Commander Devonport (NBCD) commissioned a review of the findings from the reports into the investigation to identify lessons learned. This was done in parallel with an ongoing safety culture review across the Naval Base.
"As a result, there have been improvements in the dissemination of lessons learned more widely, not only those resulting from the Minneapolis-St Paul accident.
"NBCD and (civilian contractor) Babcock have also instigated the safety initiative 'Time out for Safety' which is designed to encourage openness and improved safety culture across naval base and dockyard operations."