Initially, doctors thought Byrd was suffering from heat exhaustion and severe dehydration, but other medical conditions developed. He had been in frail health for several years.
Byrd, a Democrat, was the longest-serving senator in history. He held his seat for more than 50 years. For six of those years, he was the Senate's majority leader. Byrd was third in the line of succession to the presidency, behind House Speaker Nancy Pelosi.
Fellow West Virginian Sen. Jay Rockefeller said it was his "great privilege" to serve with Byrd.
"I looked up to him, I fought next to him, and I am deeply saddened that he is gone," Rockefeller said.
The Senate Republican leader, Mitch McConnell of Kentucky, said Byrd "combined a devotion to the U.S. Constitution with a deep learning of history to defend the interests of his state and the traditions of the Senate."
"We will remember him for his fighter's spirit, his abiding faith, and for the many times he recalled the Senate to its purposes," McConnell said.
Timeline of Sen. Robert Byrd's accomplishments:
- After six years in the West Virginia legislature, Byrd was elected to the U.S. House in 1952. Byrd entered Congress as one of its most conservative Democrats.
- In 1971, Byrd ousted Sen. Edward Kennedy of Massachusetts as the Democrats' second in command. He was elected majority leader in 1976 and held the post until Democrats lost control of the Senate four years later. He remained his party's leader through six years in the minority, then spent another two years as majority leader.
- Byrd briefly sought the Democratic presidential nomination in 1976 and later told associates he had once been approached by President Richard M. Nixon, a Republican, about accepting an appointment to the Supreme Court.
- Byrd stepped aside as majority leader in 1989 when Democrats sought a more contemporary television spokesman. "I ran the Senate like a stern parent," Byrd wrote in his memoir, "Child of the Appalachian Coalfields." His consolation price was the chairmanship of the Appropriations Committee, with control over almost limitless federal spending.
- By 1991, he surpassed his announced five-year goal of making sure more than $1 billion in federal funds was sent back to West Virginia, money used to build highways, bridges, buildings and other facilities, some named after him.
- In 2004, Byrd got Congress to require schools and colleges to teach about the Constitution every Sept. 17, the day the document was adopted in 1787.
- In 2006 and with 64 percent of the vote, Byrd won an unprecedented ninth term in the Senate just months after surpassing South Carolinian Strom Thurmond's record as its longest-serving member. His more than 18,500 roll call votes were another record.
- Byrd stood firm in his opposition against authorizing the war in Iraq. He cited Iraq when he endorsed then-Sen. Barack Obama for the Democratic presidential nomination in May 2008, calling Obama "a shining young statesman, who possesses the personal temperament and courage necessary to extricate our country from this costly misadventure."
- Byrd's accomplishments followed a childhood of poverty in West Virginia, and his success on the national stage came despite a complicated history on racial matters. As a young man, we was a member of the Ku Klux Klan for a brief period, and he joined Southern Democrats in an unsuccessful filibuster against the landmark 1964 Civil Rights Act. He later apologized for both actions, saying intolerance has no place in America. While supporting later civil rights bills, he opposed busing to integrate schools.
Full statements on passing of Sen. Byrd
"I am deeply saddened by the passing of my friend Senator Robert Byrd, whose record-breaking service greatly advanced America's pursuit of a more perfect union. Senator Byrd was the most respected authority on the rules of Congress, the Constitution, and the history of the Senate. No American leader has shown more reverence for history, for learning its lessons, and for applying them in representative government. He set an incredible example of lifetime learning - earning his college degree in his thirties and getting his law degree while in Congress, at a time when he already knew more about the law than most lawyers in the country. He wasn't afraid to learn from his mistakes or to change his mind on an issue when he felt he had been wrong. He was justly proud of the federal funds he secured to improve the lives of the people of West Virginia. And he served to the end: I will never forget watching him being wheeled on to the Senate floor to cast his decisive vote for health care reform. I am grateful for his counsel, his unfailing kindness, and his friendship. I will miss him. America will miss him. My thoughts and prayers are with his family and the people of West Virginia." - President Bill Clinton
"The Senate has lost a giant and America has lost a real fighter for our working families. Robert Byrd's eloquence during his amazing tenure in the Senate will forever be a testament to his deep reverence for the Constitution and the importance of preserving the American dream, which he lived. I will always be grateful for Senator Byrd's strong support for California - especially when our state was suffering through natural disasters - as well as his deep conviction to bring our troops home from Iraq." - Sen. Barbara Boxer (D-CA)
The Associated Press contributed to this report.