WASHINGTON (Reuters) - The chair of the National Transportation Safety Board (NTSB) said the loss of an engine cover on a Southwest Airlines Boeing 737-800 on Sunday that fell off during takeoff in Denver and struck the wing flap was the result of a maintenance issue.

NTSB Chair Jennifer Homendy told reporters after a Senate hearing the airplane had been in for maintenance the night before the incident in which it lost the engine cowling.

The board opted not to open a formal investigation, Homendy added.

Southwest Flight 3695 returned safely to Denver International airport around 8:15 a.m. local time on Sunday and was towed to the gate.