The investigation into the failure of the engine on July 28 is ongoing. The engine was built by General Electric.
NTSB investigators last week went to a GE plant in Cincinnati where the engine was built to examine it. They found that a mid-shaft fractured and the shaft is undergoing detailed study and metallurgical inspections to determine why.
Investigators are also looking at manufacturing and assembly records for the engine.
Debris from the engine fell on the runway. A small fire ignited, forcing the airport to briefly shut down. Boeing said Wednesday it continues to work with the NTSB and GE on determine what happened.