Senator Robert C. Byrd obtained secret F.B.I. documents about the civil rights movement that were leaked by the C.I.A. and set off an angry confrontation between the two agencies in the 1960s, according to newly released F.B.I. records.

Mr. Byrd, a Democrat from West Virginia who died in June 2010 at age 92, sought the intelligence because he suspected that Communists and subversives were guiding the civil rights cause, the records show. Decades before he became the longest-serving member of Congress in history, he stalled and voted against major civil rights legislation in the mid-1960s. He also belonged to the Ku Klux Klan as a young man in the 1940s, and the F.B.I. cited that membership while weighing his requests for the classified information, the records show....