New York CNN  —  A Houston-bound Boeing 737-800 plane operated by Southwest Airlines returned safely to Denver International Airport on Sunday after its engine cover fell off and struck the wing flag, according to the Federal Aviation Administration. The agency said it will investigate. This is the latest in a string of mechanical issues to plague Boeing aircraft across a range of airlines over the past several months even as the company has faced years of scrutiny regarding the safety of its planes. Boeing declined to comment and referred CNN to Southwest for information about plane and fleet operations. In a statement to CNN, Southwest said passengers would take another plane to Houston and would be approximately 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,” the statement said. Southwest said no injuries were reported. Southwest said its maintenance teams would review the aircraft, which was in the air for 35 minutes before it was forced to turn around and land. The plane was deemed airworthy in May 2015, per FAA records. Eva Rothenberg, Sarah Dewberry and Chris Boyette contributed to this report