(AP) — The CEO of United Airlines, who has been on medical leave since October, underwent a heart-transplant operation on Wednesday, and the airline said that he was recovering.

United said that it expects Oscar Munoz to return to work this spring. The airline said that a transplant was “considered the preferred treatment and was not the result of a setback in his recovery.”

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The airline did not provide more details, including where the transplant was performed. It promised to provide more information within 24 hours.

Munoz, 57, became CEO in early September after Jeff Smisek abruptly stepped down during a federal investigation into the airline’s dealings with officials who run airports in the New York City area, including Newark, New Jersey, where United has a major hub.

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Munoz suffered a heart attack in mid-October, although the company declined to confirm it for several days. At the time, United would only say that Munoz was in the hospital. On Oct. 19, United announced that Munoz would take medical leave, and general counsel Brett Hart was named acting CEO.

United is the world’s second-largest airline and a unit of Chicago-based United Continental Holdings Inc.

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