Why a Panel Tore Off an Alaska Airlines Jet Midair

A panel called a door plug may have blown off an Alaska Airlines Boeing 737 Max 9 jet shortly after takeoff on Friday because four critical bolts meant to keep it in place were missing or improperly installed, federal investigators said.

The door plug, which looks like a regular window from the inside, was placed where an emergency door would have been if the plane had more seats. The one that tore off was on the plane’s left side at Row 26. No passengers were seated next to it, federal officials said.

The blowout did not injure anyone, but it exposed passengers to powerful winds while 16,000 feet in the air and has raised questions about Boeing’s quality control. The flight on Friday was carrying 171 passengers and six crew members.

The door plug was recovered on Sunday in a Portland, Ore., backyard. The National Transportation Safety Board said the four bolts have not been found yet.

All Boeing 737 Max 9 jets with a door plug remain grounded. The Federal Aviation Administration said on Tuesday that Boeing’s instructions for how airlines should check the planes were being revised.