A broken rotor blade was cause of a helicopter crash in the North Sea which killed 11 men.

Air accident investigators say evidence from the wreckage suggests that one of the main rotor blades broke off during the flight between offshore rigs.

They are now investigating whether a lightning strike in 1999 could have caused the fatigue found in the blade.

They found no sign of any error on the part of the crew or maintenance staff.

Crew blameless

The inquiry, led by the Air Accident Investigation Branch of the Department for Transport, found "two items of major significance" among wreckage on the seabed.

These were the fractured blade, and evidence that the gearbox and the rotor head had broken away from the fuselage mountings.

Tests on the remains of the blade found signs of fatigue that showed that the "blade fracture had initiated the catastrophic event".

The investigation team said the blade had been checked by the manufacturer after being hit by lightning in 1999.

They are gathering more information about the lightning strike.

They have not yet uncovered any evidence of mistakes made by either the helicopter crew or the maintenance staff.

'Catastrophic event'

The investigators said flight and acoustic data gave no sign the pilots had been aware of any problems.

The Civil Aviation Authority is now considering if any safety action needs to be taken.

The aircraft, owned by Bristow helicopters, went down about 30 miles off the coast on 16 July.

It was carrying two pilots and nine passengers.

Ten bodies have been recovered, while the search for the remaining missing person was called off on Tuesday.

The air accident report described the accident as "not survivable".