Less than a year ago, both the Federal Aviation Administration and the engine's manufacturer drew attention to problems with metal fatigue on the fan blades in the CFM56 engine series, which this plane has, after a similar incident in 2016, also a Southwest plane. The FAA proposed a directive that planes with extensive miles on their engines should subject the fan blades on those engines to a specific test designed to detect flaws in metal. Whether this plane's engines should have been subjected to those tests, and whether Southwest was performing them, will be investigated, Sumwalt said.