Robert Byrne passed away yesterday (4/12) at the age of 84. The USCF has a brief write-up
here.
Until browsing online just now, I didn't realize he was such a strong player. Chessmetrics has him as #11 in the world in the 1973/74 time frame. As the USCF article shows, he beat Fischer in 1965. His best tournament performance was Leningrad 1973 where he finished 3rd, scoring 12.5/17 (2793 performance rating, see results
here) and finishing just behind Korchnoi and Karpov who tied for first. He finished ahead of names like Larsen, Hubner, Tal, Taimanov, and Gligoric. Pretty impressive.