WASHINGTON, Jan 14 (Reuters) - The National Transportation Safety Board said on Wednesday that a key part ‌of a UPS MD-11 cargo jet ​that crashed in November ‍in Kentucky, killing 15 ⁠people, ⁠that was found to have cracks ‌was the subject ​of a 2011 Boeing ( BA ) service letter.

The NTSB ⁠said its investigation ‍to ​date has found fatigue cracks in a support structure ‍on the left pylon, which connects to the wing and the plane's engine, known as the ​bearing ‍race.

Boeing's ( BA ) 2011 letter said there had been four ​previous failures of the bearing race in three airplanes but did not consider it to be a safety of flight ​issue, the NTSB said. The MD-11 remains grounded since the crash.