The FAA said in a statement that Southwest Airlines Flight 3695, a Boeing 737-800 passenger jet, landed back at Denver International Airport "after the crew reported the engine cowling fell off during takeoff and struck the wing flap."
The plane, which took off at 8.15 am Sunday (1415 GMT) and was bound for Houston, was towed to the gate after landing and the FAA would investigate the incident, the US aviation authority said.
CNN, ABC and other US outlets broadcast dramatic videos taken by passengers that showed what appeared to be part of the cowling hanging off and flapping in the wind.
"Let's go ahead and declare an emergency for Southwest 3695 and we'd like an immediate return," a crew member said, according to The New York Times, which cited radio transmissions with an air traffic controller. "We've got a piece of the engine cowling hanging off."
Southwest described the incident as a "mechanical issue" and said the flight had landed safely with passengers rebooked on other flights.
"We apologized to those Customers for the inconvenience of their delay and our Maintenance teams are reviewing the aircraft. We place our highest priority on ultimate Safety for our Customers and Employees," a spokesman told AFP via email.
Boeing declined to comment publicly on the incident.
The aircraft maker has suffered a series of safety issues in recent years, including the fatal Lion Air and Ethiopian Airlines crashes of 737 MAX planes in 2018 and 2019 that killed more than 350 people.
The manufacturer is also still reeling from a near-catastrophic incident in January when a fuselage panel on a Boeing 737 MAX 9 Alaska Airlines jet blew off mid-flight in the United States.
In March, a Boeing 777 jetliner bound for Japan had to make an emergency landing when a wheel fell off during take-off and landed in a San Francisco International Airport parking lot.
Last month US regulators gave Boeing 90 days to come up with a plan addressing quality control issues, with the FAA chief saying the company must "commit to real and profound improvements".
The headline-generating safety issues have left Boeing a frequent punching bag on social media. "'When one door closes, another one opens.' - Boeing" read just one of many popular jokes and memes about the manufacturer.
Meanwhile, Texas attorney general launched an investigation last month into Boeing supplier Spirit AeroSystems over recurring issues with some parts it supplies the US aviation giant.
Spirit AeroSystems manufactures fuselages and other large airplane parts, including for Boeing's 737 jets.
The announcement by Texas Attorney General Ken Paxton came only days after Boeing said some of its top executives, including CEO Dave Calhoun, were leaving the company.
