WASHINGTON (Reuters) - Boeing ( BA ) said on Friday it believes required documents involving the opening and closing of a key part during production of a 737 MAX 9 that failed during a mid-air emergency were never created, according to a letter seen by Reuters.

The National Transportation Safety Board said last month the door plug that flew off an Alaska Airlines 737 MAX 9 jet mid-flight on Jan. 5 appeared to be missing four key bolts.

Boeing ( BA ) told U.S. Senator Maria Cantwell in a letter seen by Reuters that "we have looked extensively and have not found any such documentation" and the planemaker's working hypothesis is "the documents required by our processes were not created when the door plug was opened."