Sen. Robert Byrd (D-W.Va.) is “seriously ill” at a Washington-area hospital, according to a statement released Sunday by his office. He is 92.

“Byrd was admitted to the hospital late last week suffering from what was believed to be heat exhaustion and severe dehydration as a result of the extreme temperatures,” the statement said.

He “was expected to remain in the hospital not more than a few days,” the statement added. But “upon further examination by his doctors, other conditions have developed which has resulted in his condition being described as ‘serious.'”

Byrd is the Senate’s longest-serving Member and its President Pro Tem. He battled a staph infection last year that kept him hospitalized for six and a half weeks.