American Express Co. President Ed Gilligan, who died last week on a return trip from Tokyo, suffered an embolism and subsequent heart attack, according to a person briefed on the matter.

Flight attendants on the corporate jet en route to New York May 29 performed cardiopulmonary resuscitation on Gilligan, 55, before the plane made an emergency landing in Green Bay, Wisconsin, said the person, who asked not to identified discussing personal matters. Two other executives were on board.

Blood clots, also called deep-vein thrombosis, can be a serious risk for long-distance travelers, according to the U.S. Centers for Disease Control & Prevention’s website. The longer a traveler is immobile, the greater the risk of developing a clot.

Gilligan began working as an intern at Amex 35 years ago while a student at New York University. He was named vice chairman in 2007 and president in 2013, and oversaw digital initiatives, including a partnership reached last year with car- service firm Uber Technologies Inc.

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