American Express Co. President Ed Gilligan died Friday morning after becoming ill on a flight to New York. The cause of death is believed to be a heart attack, a company spokesperson said.

"This is deeply painful and frankly unimaginable for all of us who had the great fortune to work with Ed, and benefit from his insights, leadership and enthusiasm," Chief Executive Officer Ken Chenault said in a letter to the staff. "His contributions have left an indelible imprint on practically every area of our business."

Gilligan, 55, began working at the company about 35 years ago, and was named president in 2013, according to the letter. He is survived by his wife, Lisa, and their children — Katie, Meaghan, Kevin and Shane, the company said.

People who reported to Gilligan will now report to Chenault, the letter said.

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