DENVER (KDVR) — A Southwest Airlines flight safely returned to Denver International Airport early Sunday after a removable piece of metal that covers the engine fell off and struck a wing flap, according to the Federal Aviation Administration.

The Boeing 737-800 plane was departing for William P. Hobby Airport in Houston at around 8:15 a.m. local time when the engine cowling fell off. Video posted to social media shows the metal segment flapping against the wing while it was still on the tarmac.

The Southwest flight was towed to the gate after landing.

Southwest Airlines told Nexstar’s KDVR that customers on the original plane were transferred to another aircraft that was set to arrive in Houston around three hours behind schedule.

“We apologize for the inconvenience of their delay, but place our highest priority on ultimate Safety for our Customers and Employees,” it said in a statement. “Our Maintenance teams are reviewing the aircraft.”

The exact number of customers and crew on the flight was not known, and the FAA was investigating the incident.

This is the fourth time a Boeing plane has had to divert to Denver for issues so far this year.

On March 29, engine issues forced a United Airlines flight from San Francisco to Paris to divert to Denver. Crew members on the Boeing 777-200 plane reported having an issue with one engine.

In February, another United Airlines flight, this time from San Francisco to Boston, safely landed at DIA after issues with the wing were reported mid-flight. The plane was a Boeing 757-200.

In January, a United Airlines flight from Las Vegas to Washington, D.C. was diverted to Denver due to a cracked windshield.