Manufacturers say the problem is a "one-off"
Investigators say damage to a Lincolnshire wind farm was not caused by a UFO or any other aircraft.
A turbine at Conisholme lost one 66ft (20m) blade and another was badly damaged on 4 January.
UFO enthusiasts claimed it was caused by a mystery aircraft but scientists now say a collision was not responsible.
An exact cause has not yet been determined but it is thought to be "material fatigue".
Dale Vince, managing director of wind farm operator Ecotricity, said: "They [scientists] looked at all the broken parts of the turbine, the parts that were left standing at the top and examined the land around the bottom of the turbine looking for debris.
"But it was actually by examining the ring of bolts that hold the blade on that the examiners were able to say definitely it wasn't a collision that caused this problem."
Mr Vince said he expected a full explanation into the cause of the damage in two weeks.
"There is a ring of about 30 bolts and they exhibit what examiners term as classic fatigue signs.
"They've ruled out bolt failure and are looking for the cause either side of the bolts in one of the components."
He said manufacturers had checked 1,000 turbines of the same design all over the world and it was thought the problem was a "one-off".
The turbine is one of 20 at the Conisholme site, which has been fully operational since April 2008.
Residents had reported seeing flashing lights around the time the turbine was damaged.