The cause was congestive heart failure, said his son, Burke Doar.
Doar was a Justice Department civil rights lawyer from 1960 to 1967, serving under the administrations of President John F. Kennedy and Lyndon B. Johnson and rising to the position of assistant attorney general in charge of the Civil Rights Division.
Working for the federal government at the height of the Civil Rights Movement, Doar challenged discriminatory policies in Southern states that curtailed minority access to the voting booth and state universities.
Doar later was the lead prosecutor in the federal trial arising from the deaths of three civil rights workers, Andrew Goodman, James Chaney and Michael Schwerner. Those killings inspired the 1988 film "Mississippi Burning."
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Later in his career, he served as special counsel to the House of Representatives as it investigated the Watergate scandal, where he recommended the impeachment of President Richard Nixon.
Among the lawyers on the impeachment committee staff was Hillary Rodham, now Hillary Rodham Clinton, the former secretary of state.
In a 2009 interview with C-SPAN, Doar described the election of Obama as "rewarding" and marveled at the progress toward racial equality since 1960.