The man who shot a congressman and four other people had apparently volunteered for Bernie Sanders' 2016 presidential campaign, the Vermont senator said before assailing violence as "unacceptable in our society."
"I am sickened by this despicable act," Sanders said on Wednesday.
Sanders, 75, unsuccessfully sought the Democratic nomination against Hillary Clinton. Sanders, an independent, ran a passionate, anti-establishment campaign that won support from many younger, liberal voters and gave Clinton a closer race than many expected, but he eventually backed her candidacy.
The senator spoke hours after a man identified as James Hodgkinson of Belleville, Illinois, opened fire on Republican lawmakers, aides and others practising baseball in suburban Alexandria, Virginia. Officers shot Hodgkinson, who later died.
Sanders issued a two-paragraph statement on the shooting and read it nearly verbatim on the Senate floor. He ignored a reporter's attempt to ask him questions.
"Violence of any kind is unacceptable in our society, and I condemn this action in the strongest possible terms," Sanders said. "Real change can only come about through nonviolent action, and anything else runs against our most deeply held American values."
Robert Becker, director of Sanders' Iowa campaign, said staff workers could not recall Hodgkinson. Hodgkinson was said to have been among 1,000 volunteers around Davenport, but Becker said, "Nobody has a recollection of this guy coming in."
Conservatives immediately seized on Hodgkinson's support for Sanders, with commentator Ann Coulter, a frequent voice on conservative cable news and talk radio, describing the gunman as "some nut Bernie Sanders supporter."