American Express said president Ed Gilligan died suddenly Friday after falling ill while flying on a corporate plane to New York.

The credit card company said the 55-year-old executive was coming back from a business trip. The plane made an emergency landing in the U.S., an American Express spokeswoman said, but she declined to say where Gilligan had been.

"This is deeply painful and frankly unimaginable for all of us who had the great fortune to work with Ed," said American Express CEO Ken Chenault, in a letter to employees Friday.

Gilligan began working at American Express Co. 35 years ago as an intern. He was named president of the company in 2013 and reported to Chenault.

American Express said he is survived by his wife and four children.