U.S. NTSB investigators are on scene examining damage to the engine of the Southwest Airlines plane in this image released from Philadelphia, Pennsylvania, April 17, 2018.

A piece of engine covering that broke lose during a mid-air engine failure on a Southwest Airlines flight last month may have contributed to the damage, federal investigators said Thursday.

One of the fan blades on the Boeing 737-700's engines broke off shortly into Flight 1380, when the plane was above 30,000 feet. Debris flew, puncturing the fuselage and blowing out a window. A passenger was partly sucked through the opening and died, marking Southwest's first accident-related passenger fatality in 47 years of flying and the first such fatality on a U.S. commercial airline since 2009.

The captain made a safe emergency landing in Philadelphia after the cabin depressurized when the window broke.

The National Transportation Safety Board on Thursday reiterated that it appeared metal fatigue in the fan blade was to blame for it breaking off.

But the fan blade did not appear to be responsible for busting the window.

"A large gouge impact mark, consistent in shape to a recovered portion of fan cowl and latching mechanism, was adjacent to the row 14 window," it said in a report. "No window, airplane structure, or engine material was found inside the cabin."

Photo: NTSB

Southwest COO Mike Van de Ven last week said on an earnings call that the blade alone likely did not cause the damage.

Federal regulators have mandated more stringent testing of the fan blades, which can crack after extensive use. Southwest expects to complete this task by the middle of May.

But Van de Ven said the company is also interested in the role of the engine cowling.

"What we are talking about is whether or not there are opportunities in the inlet cowling to improve durability so that it can minimize the kind of energy that comes out with this fan blade release," he said.