Last January, a door plug broke off on an Alaska Airlines plane. The National Transportation Safety Board said last month that the door plug was missing four key screws.

“We searched extensively and did not find any such documents,” Boeing Executive Vice President Ziad Ocakli told Senator Maria Cantwell in the letter. Yesterday, a Boeing 777 passenger plane was forced to make an emergency landing shortly after taking off from a San Francisco airport on a trip to Japan, after one of its tires fell in the airport parking lot.