A United Airlines flight landed with a missing panel from the plane, believed to have fallen mid-flight.

The plane was a Boeing 737 that left San Francisco International Airport for Mogue Valley International/Medford Airport in Medford, Oregon Friday around 10:20 a.m.

After the plane was parked, an external panel was discovered to be missing, according to a spokesperson with United Airlines.

The plane was carrying 139 passengers with six crew members.