WASHINGTON, March 8 (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."