WASHINGTON - Sen. Daniel Inouye of Hawaii, 88, the influential Democrat who broke racial barriers on Capitol Hill and played key roles in congressional investigations of the Watergate and Iran-Contra scandals, died Monday.

Sen. Inouye, in office since January 1963, was currently the longest-serving senator and was president pro tempore of the Senate, third in the line presidential succession. His office said Monday that he died of respiratory complications at a Washington-area hospital.

Less than an hour after Sen. Inouye's passing, Majority Leader Harry Reid announced the death to a stunned chamber.

"Our friend Daniel Inouye has died," Reid said somberly. Shocked members of the Senate stood in the aisles or slumped in their chairs.

Sen. Inouye was a World War II hero and Medal of Honor winner who lost an arm to a German hand grenade during a battle in Italy. He became the first Japanese American to serve in Congress, when he was elected to the House in 1959, the year Hawaii became a state.

He won election to the Senate three years later and served there longer than anyone in American history except Robert Byrd of West Virginia, who died in 2010 after 51 years in the Senate.

President Obama, a native of Hawaii, said in a statement, "Tonight, our country has lost a true American hero with the passing of Sen. Daniel Inouye."

His last utterance, his office said, was "Aloha."