By David Shepardson and Mrinmay Dey

(Reuters) -The Federal Aviation Administration is investigating how a United Airlines Boeing (NYSE:BA) 737-800 lost an external panel before landing safely in Oregon Friday.

United Flight 433 landed at Medford Airport around 1:45 p.m. PDT (2045 GMT) 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 airplane revealed a missing panel. United said it will also investigate and said no emergency was 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 in a statement.

Boeing did not comment, directing questions to United Airlines. According to FAA records, the plane was built in late 1998.

The Rogue Valley Times posted a dramatic photo of the airplane with the missing panel. Traffic was briefly halted at the airport to search for the panel.

In the aftermath of an Alaska Airlines Boeing 737 MAX 9 mid-air emergency prompted by a door plug blowout, U.S. aviation incidents are getting more attention.

Boeing has been under heavy regulatory scrutiny following the Jan. 5 Alaska Airlines incident, with probes into the company's safety and quality standards in its production process.

Last week a United Airlines-operated Boeing 737 MAX rolled onto the grass and off the runway in Houston, prompting investigations.

Also last week a United-operated Boeing 777-200 bound for Japan lost a tire after takeoff from San Francisco and was diverted to Los Angeles where it landed safely.

A United Boeing 737 on March 4 bound for Florida departing Houston returned to the airport shortly after takeoff after the engine ingested some plastic bubble wrap that was on the airfield prior to departure. Social media posts showed flames coming out of the engine.