Amid a federal investigation into what caused a door plug to blow off a Boeing 737 Max 9 jet in January, a Boeing top official said the company cannot find documents about the door plug and it's likely that such records never existed.

Ziad Ojakli, the executive vice president of government operations at Boeing, said employees looked "extensively" and failed to turn up any paperwork about the "opening and closing of the door plug," he said in a letter to Washington Sen. Maria Cantwell on Friday.

According to Ojakli, Boeing's working hypothesis is that "the documents required by our processes were not created when the door plug was opened."

The letter was in response to the Senate Commerce Committee's hearing on Wednesday, where National Transportation Safety Board chair Jennifer Homendy accused Boeing of withholding key information about the door plug.

Ojakli denied the allegations and said the company has been committed to cooperating "fully and transparently" ever since the incident.

In a preliminary report released last month, the NTSB said the door plug in question was missing four key bolts — ones that help keep the door plug in place. Investigators believe the bolts were not re-installed while the plane received some repair work at Boeing's factory in Washington state last year.

On Wednesday, Homendy said the NTSB still does not know who was responsible for failing to reattach the door plug properly.

Ojakli responded in his letter that Boeing has since sent over additional names of employees as requested.

Boeing has been under intense scrutiny ever since Jan 5., when a door plug tore off at 16,000 feet and left a massive hole during an Alaska Airlines flight. No one was seriously injured.

The NTSB and the Federal Aviation Administration have both launched investigations into the company. The Department of Justice recently opened a criminal probe into the incident, The Wall Street Journal reported on Saturday.

Boeing did not immediately respond to NPR's request for comment.

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