By David Shepardson

WASHINGTON (Reuters) - The head of the National Transportation Safety Board said on Wednesday that Boeing has not provided some documents and information sought in its ongoing investigation into the Jan. 5 Alaska Airlines 737 MAX 9 mid-air cabin door emergency.

NTSB Chair Jennifer Homendy said investigators have sought the names of the 25 people who work on door plugs at a Boeing facility in Renton, Washington, but have not received them from Boeing. "It is absurd that two months later we don't have it," Homendy said at a Senate Commerce hearing.

"It's not for lack of trying," she said, adding that the information being sought includes the precise shift that worked on the improperly installed door plug in September. The agency also seeks documentation related to opening and closing of the door plug and removal of key bolts.

Boeing did not immediately comment.

The NTSB said last month four key bolts attaching the plug to the fuselage appeared to be missing.

The planemaker has scrambled to explain and strengthen safety procedures since the January mid-air incident that led to the FAA grounding the MAX 9 for several weeks. The company has been the subject of increased scrutiny from regulators and big air carriers concerned about the quality of jet production.

Homendy said investigators began interviews at Boeing's Renton plant on Sunday and that they will continue all week.

Senator Ted Cruz, the top Republican on the Commerce Committee, called it "utterly unacceptable" that the NTSB is not receiving full cooperation from Boeing.

Senate Commerce Committee Chair Maria Cantwell said she planned to write a letter asking about the lack of cooperation.

Boeing's CEO, Dave Calhoun, told employees in January: “We’re going to approach it with 100% and complete transparency every step of the way. We are going to work with the NTSB who is investigating the accident itself to find out what the cause is."

(Reporting by David Shepardson in Washington; Editing by Matthew Lewis)