Flying on United or Alaska Airlines after their Boeing 737 Max 9 jets were grounded? Here's what to know.

The ongoing investigation into a door plug that blew out mid-flight on an Alaska Airlines flight is continuing to impact travelers, with both Alaska and United Airlines cancelling hundreds of flights due to the grounding of their fleets of Boeing 737 Max 9 jets.

The airlines are the only two U.S. passenger airlines that operate flights on the Boeing 737 Max 9 aircraft, the jet involved in the mid-air blowout. In the aftermath of the incident, both airlines say they have found loose hardware on door plugs on several Boeing 737 Max 9 planes.

The Federal Aviation Administration said all Boeing 737 Max 9 planes in the U.S. will be grounded until the agency is "satisfied that they are safe," an agency spokesperson said in a statement Sunday.