(Reuters) -United Airlines on Friday said one of its Boeing 737-800 aircraft was found to have a missing external panel after landing safely at its scheduled destination at Rogue Valley International Medford Airport in Oregon.

United said the plane was carrying 139 passengers and six crew, and the missing panel was discovered after the plane was parked at the gate.

"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, adding it will also investigate to understand the cause of the incident.

Boeing did not comment, directing questions to United Airlines.

The plane had departed from San Francisco, according to flight tracking website FlightRadar24.

Last week a United Airlines-operated Boeing 737 MAX rolled onto the grass and off the runway at George Bush Airport in Houston, and the incident was being investigated by the National Transportation Safety Board and the Federal Aviation Administration.

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