Southwest spokeswoman Brandy King said results of the reviews were handed over to the manufacturer, CFM, and did not say whether the engines were found to be safe or unsafe.
A statement from CFM spokeswoman Jamie Jewell did not provide any information on what the inspections showed.
"We cannot speak to the Southwest fleet or other airlines, for that matter," Jewell wrote.
A Southwest-operated 737 Max experienced an engine problem in late March while being ferried from Florida to California for storage. The pilots turned around for an emergency landing after reporting "a performance issue with one of the engines shortly after takeoff," Southwest said at the time.
The Federal Aviation Administration ordered the planes pulled from service last month after two fatal crashes related to a stabilization system and faulty sensor readings.
The Wall Street Journal first reported the engine inspections.
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