An explosion on a nuclear submarine which killed two British sailors was caused by an emergency oxygen device, the US military has said.
The men died on board the Devonport-based submarine HMS Tireless during a joint British-US operation under the Arctic ice off Alaska.
A third sailor was airlifted to a US military hospital in Anchorage. He is expected to make a full recovery.
The boat had to smash through the ice to reach the surface after the blast.
A Board of Inquiry investigation has begun into the accident, which took place on Wednesday morning.
Tireless is a nuclear-powered Trafalgar-class submarine. It does not carry nuclear weapons, but is armed with five tubes capable of firing Tomahawk missiles.
It is the third of seven such vessels in the Royal Navy, and usually carries a crew of 130.
US authorities said air-purification equipment, known as a self-contained oxygen generation candle, was the source of the blast.
Oxygen candles are emergency devices that create oxygen through a chemical reaction.
Some devices burn at high temperatures during the reaction.
HMS Tireless was launched in 1985 - the piece of air-purification machinery thought to have failed was fitted as part of an update in 2001.
The air purification system, which does not provide the submarine's main oxygen supply, is in the forward escape compartment - a "quiet" area at the front of the vessel, where submariners go to relax when they are not on duty.
A Ministry of Defence (MoD) spokesman said its use on other vessels had been restricted as a precaution until safety checks were carried out.
Oxygen candles are fitted to all Trafalgar-class submarines.
Class - Trafalgar
Commissioned - 1984
Crew - 130
Weapons - Tomahawk missiles, Spearfish torpedoes
Propulsion - pressurised water-cooled nuclear reactor
Speed - 32knots (dived)
Depth - 400m (operational), 600m (maximum)
Length - 85.4m (280ft)
The former First Sea Lord, Admiral Sir Alan West, told Radio 4's Today programme there had been no previous problems with the equipment.
The nuclear reactor of HMS Tireless was unaffected and the ship itself was not in danger, the MoD said.
The Leader of the Commons Jack Straw told MPs: "Those of us who have been on board submarines know just what a potentially dangerous environment it is, and the safety record of the Royal Navy overall was second to none.
"We send our sympathy to the family and the colleagues of the two sailors who were killed."
The families of both dead sailors have been told.
Vice Admiral Jay Donnelly, commander of the US submarine force, said: "I am deeply saddened at the loss of the crew members from the Tireless.
"Submariners are brothers at sea and we all feel the loss as if it were our own. We stand by to continue to assist in any way we can."
The Board of Inquiry aims to prevent a recurrence of the tragedy and to see what lessons can be learned.