An engine cowling fell off of a Southwest Airlines Boeing 737-800 and struck a wing flap during takeoff from Denver International Airport, the Federal Aviation Administration said Sunday.

The FAA said Southwest Flight 3695 was on its way to Houston’s William P. Hobby Airport and safely returned to the gate at Denver at 8:15 a.m. local time. Southwest said customers on the flight transferred to a different aircraft and were scheduled to arrive at their destination three hours late.

“Our Maintenance teams are reviewing the aircraft,” Southwest said. The FAA said it is investigating the incident. Southwest didn’t immediately respond when asked when the plane and engine last underwent maintenance.

In response to a request for comment, Boeing pointed to Southwest’s statement.

The cowling loss comes as the FAA investigates a separate Southwest incident in March. One of its flights strayed off course and flew close to the air traffic control tower at LaGuardia Airport as it attempted a landing in New York.

The plane is an older model of the Boeing 737 than the Max jets. Boeing is under heightened regulatory scrutiny after a January incident when a door plug blew off a nearly new 737 Max 9 when the Alaska Airlines flight was at 16,000 feet, causing a near-catastrophe.

Boeing’s quality control issues have spiraled into safety concerns, slowing deliveries of new Max aircraft. Big Boeing customers like Southwest and United say the issues have affected their growth plans.

The long-awaited FAA certification of its 737 Max 7 and Max 10 models is also behind previous schedules. Boeing CEO Dave Calhoun last month said that he would step down by year’s end, and Boeing replaced its chairman and chief executive of its commercial airplane unit.