The incident, at what is Europe's largest wind farm, happened in the early hours of March 19. The plant, which is located in southern Scotland, automatically switched itself off.
Siemens said an examination of the broken blade revealed a "material defect in the laminate". The company said there could be the result of changes to the manufacturing process.
A Siemens spokesman said: "We are investigating a link between an adjustment in the production process relevant to a specific position of the B45 blade, and a manufacturing error.
"The adjustment was implemented shortly before the blade in question was manufactured. As a precautionary measure we will carry out inspections of the specific position on all B45 blades produced within a certain period after this date. However, we do not expect to find further defects of this type."
All of the Whitelee turbine blades has been investigated and are back online. No one was hurt as a result of the original accident.
Blade failure on turbines is relatively rare. In one recent incident a blade split on a Gamesa 2.5MW turbine at the 50MW Kumeyaay wind plant, owned by Australia's Infigen Energy, an offshoot of now defunct Babcock & Brown.
In 2008 Vestas was confronted with a blade failure when a 23 metre blade came off an eight-year-old V47 660 kW machine installed at Windpark Tuitjenhorn in the province of Noord-Holland.