Donald G. Triplett, known as “Case 1,” the first person in the U.S. to be diagnosed with the contemporary definition of autism, has died at age 89.

Triplett worked for 65 years at a bank in the small town of Forest, Miss., reports PBS.org. His father, Beamon Triplett, was a primary shareholder. Donald Triplett graduated in 1958 from Millsaps College, was a golfer and traveler who went to some exotic destinations, bank CEO Allen Breland told PBS.

Breland, who also spoke with WLBT.com, said Triplett was “in his own world, but if you gave him two, three-digit numbers, he could multiply them faster than you could get the answer on a calculator.”

WLBT.com reported that his autism diagnosis was the result of a “meticulously detailed 22-page letter his father composed when Donald was just a child. The letter was full of telling observations that Beamon and his wife, Mary Triplett, had made of their son’s aptitudes and behavior. It was received by a Johns Hopkins researcher in Baltimore with great interest. The letter remains a primary reference document for those who study autism.”

Triplett was the subject of a book, In a Different Key, and a PBS documentary film.