A United Airlines flight that took off on Friday morning from San Francisco International Airport landed in Oregon missing an external panel, the Federal Aviation Administration said.

The panel was found to be missing after the plane, a Boeing 737-800, landed safely at its scheduled destination at Rogue Valley International Medford Airport in Oregon and parked at a gate, United Airlines said in a statement. It was unclear when or how the panel went missing.

According to the airline, there was no indication of any damage to the plane during the flight, and the aircraft did not declare an emergency on its way to the Medford airport.

“We’ll conduct a thorough examination of the plane and perform all the needed repairs before it returns to service,” the airline said. “We’ll also conduct an investigation to better understand how this damage occurred.”

The plane was carrying 139 passengers and a crew of six, according to United Airlines. No injuries were reported.

The plane has been in service for more than 25 years, and it was from a previous generation of 737 aircraft, according to Airfleets.net, a website that tracks aircraft information.

The airport briefly paused operations to inspect the runway, and resumed flights after no debris was found on the airfield, Amber Judd, the director of the Medford airport, said in an email.

Boeing referred questions about the flight to United Airlines. The F.A.A. said it planned to investigate the episode.

The discovery of the missing panel on Friday came as Boeing has faced heavy scrutiny in recent weeks after a door-sized section blew off a Boeing 737 Max 9 Alaska Airlines flight in January just minutes after it had taken off from Portland, Ore. There were no major injuries during the flight, but the frightening episode, which was recorded on video, prompted government officials to look into quality control at Boeing.

After the January flight, the F.A.A. began a six-week audit of Boeing, which found “multiple instances” in which the plane maker had failed to follow through with quality-control requirements.

Since then, there have been a number of issues with flights on Boeing aircraft.

On March 8, a United Airlines flight that had landed at George Bush Intercontinental Airport in Houston rolled into the grass as the plane, a Boeing 737, exited onto the taxiway, according to the F.A.A.

In February, a Madrid-bound American Airlines flight, a Boeing 777, diverted to Boston Logan International Airport with a cracked windshield shortly after it had departed from Kennedy International Airport in New York.