Boeing’s design of a part that blew off a jetliner is being investigated by the federal government

Federal officials are investigating whether Boeing failed to ensure that a panel that blew off a jetliner mid-flight last week was properly designed and safe to use.

The Federal Aviation Administration said Thursday that the investigation focuses on plugs used to fill spots for additional exits when those doors are not needed on Boeing 737 Max 9 planes.

The plug that blew an Alaska Airlines Max 9 was found near Portland, Oregon, and will be examined in a government laboratory.

“This incident should never have happened and it cannot happen again,” the FAA said in a statement. “Boeing’s manufacturing practices must meet the high safety standards they are legally required to meet.”

The FAA informed Boeing of the investigation in a letter dated Wednesday. The agency asked the aircraft manufacturer to submit evidence or statements in its defense within ten working days.

The FAA has grounded Max 9 jets, including all 65 from Alaska and 79 from United Airlines, until Boeing can develop inspection guidelines and planes can be examined. Alaska has canceled all Max 9s flights through Saturday.

National Transportation Safety Board investigators said this week that they have been unable to find four bolts used to secure the 63-pound door plug. They’re not sure if the bolts were there before the plane took off.