The US Federal Aviation Administration is investigating how a United Airlines Boeing 737-800 lost an external panel before landing safely in Oregon.

United flight 433 landed at Medford airport at about 1.45pm on Friday carrying 139 passengers and six crew after departing from San Francisco, the FAA and airline said.

The FAA said a post-landing airline inspection of the 25-year-old plane revealed a missing panel. United said it would also investigate. It said no emergency had been declared because there was no indication of the damage during flight.

“We’ll conduct a thorough examination of the plane and perform all the needed repairs before it returns to service,” the airline said.

Boeing did not comment, directing questions to United Airlines.

FAA records show the plane was built in late 1998. The Rogue Valley Times posted a photo of the plane with the missing panel. Traffic was briefly halted at the airport to search for it.

US flight incidents are getting more attention after a door plug blew off an Alaska Airlines Boeing 737 MAX 9 in mid-air. Boeing has been under heavy regulatory scrutiny since the 5 January incident, with investigations under way into the company’s safety and quality standards in its production process.

Last week a United Airlines-operated Boeing 737 MAX rolled off the runway in Houston, prompting investigations; and a United-operated Boeing 777-200 bound for Japan lost a tyre after takeoff from San Francisco and was diverted to Los Angeles where it landed safely.

A United Boeing 737 bound for Florida from Houston on 4 March returned to the airport shortly after takeoff because the engine had taken in plastic bubble wrap that was on the airfield before departure. Social media posts showed flames coming out of the engine.