A Southwest Airlines pilot is under an internal investigation after signing off from a flight using the phrase "Let's go, Brandon," the airline said in a statement provided to NPR.

With its origins at a NASCAR race in Alabama in October, the phrase "Let's go, Brandon" has become common in conservative circles as a way of saying, "F*** Joe Biden." It started as a meme that has now widely spread to Republican members of Congress and has even been said on the House floor.

The Associated Press reported that the pilot used the phrase on a flight from Houston to Albuquerque, N.M., and that there were audible gasps from passengers.

Southwest says it does not condone its employees sharing personal political opinions while working.

"One Employee's individual perspective should not be interpreted as a viewpoint for Southwest and its collective 54,000 Employees," the airline's statement said.

"Southwest is conducting an internal investigation into the recently reported event and will address the situation directly with any Employee involved while continuing to remind all Employees that public expression of personal opinions while on duty is unacceptable. Southwest does not tolerate any behavior that encourages divisiveness," the airline said.

This story originally appeared on the Morning Edition live blog.

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