- The Alaska Airlines Boeing 737 Max 9 returned to Portland 35 minutes into its flight to California after an outer section, including a window, fell.
- The US Federal Aviation Agency said the plane "returned safely... after the crew reported a pressurisation issue".
A passenger plane lost a section of its fuselage in mid-air, forcing it to make an emergency landing in the US state of Oregon on Friday.
The Alaska Airlines Boeing 737 Max 9 returned to Portland 35 minutes into its flight to California after an outer section, including a window, fell.
The US Federal Aviation Agency said the plane "returned safely... after the crew reported a pressurisation issue".
Alaska said 177 passengers and crew were on board.
Boeing said it was aware of the incident and was "working to gather more information".
"A Boeing technical team stands ready to support the investigation," it said.
According to flight tracking websites Flightaware and FlightRadar24, the plane was a Boeing 737 Max 9.
Alaska Airlines said the incident happened shortly after departure and that the plane had landed safely back in Portland.
"While this type of occurrence is rare, our flight crew was trained and prepared to safely manage the situation," the airline said.
It was flying at more than 16,000ft (4,876m) when it began its emergency descent, according to flight tracking data.
Images sent to news outlets show the night sky visible through a gap in the aircraft's fuselage, with insulation material and other debris also seen.
Other pictures show the seat closest to the affected section, a window seat that passengers said was unoccupied, leaning forward without its cushion.
According to the photographs, the affected area was in the back third of the plane, behind the wing and engines.
The section of fuselage involved appears to be an area that can be used as an additional emergency exit door by some operators of the aircraft type, but not by Alaska.
The National Transportation Safety Board (NTSB) wrote on X, formerly Twitter, that it was investigating the incident.
The Boeing 737 Max has been described as "the most scrutinised transport aircraft in history" after a series of safety issues and investigations.
The aircraft type was grounded in March 2019 for a year-and-a-half after two of the type crashed in similar circumstances, killing those on board.
To fly again, each Max plane underwent significant modifications, although the changes would not be visible from the outside and passengers would not notice any difference.
More recently, Boeing said it would increase the pace of 737 Max deliveries after resolving a supply error that required it to conduct lengthy inspections of new planes and its inventory, Reuters news agency reported.
About 1,300 737 Max aircraft have been delivered to customers, Boeing data shows.
Last month, the Federal Aviation Administration (FAA) urged airlines to inspect Max models for a possible loose bolt in rudder control systems.