A rifle-wielding critic of President Donald Trump opened fire Wednesday on US lawmakers practicing for a charity baseball game, seriously wounding a top Republican congressman and three others before he was killed by police.
The 66-year-old gunman was identified as James T. Hodgkinson of Belleville, Illinois, an ardent supporter of liberal Senator Bernie Sanders -- who said he was "sickened" by the "despicable act."
The attack provoked a wave of emotion on Capitol Hill, where Republicans and Democrats expressed concern over a possible rise in verbal or physical violence amid a bitterly partisan political atmosphere in the aftermath of a vitriol-filled 2016 election.
In a national address, Trump appealed for unity following the morning assault in the Washington suburb of Alexandria, Virginia.
"We are strongest when we are unified and when we work together for the common good," said the president.
Congressman Steve Scalise -- the number three Republican in the House of Representatives and a strong supporter of US gun rights -- underwent surgery after being shot in the left hip.
He needed several blood transfusions, remains in critical condition and will require more operations, MedStar Washington Hospital Center said late Wednesday.
"The bullet traveled across his pelvis, fracturing bones, injuring internal organs, and causing severe bleeding," a statement said.
The others shot and wounded were a congressional aide, a lobbyist for Tyson Foods, and a US Capitol Police officer.
A second Capitol police officer suffered non-gunshot injuries, the FBI said. Two congressman said they had sustained minor injuries while trying to take cover.
The president and First Lady Melania Trump later visited Scalise, bringing flowers.
"Just left hospital. Rep. Steve Scalise, one of the truly great people, is in very tough shape - but he is a real fighter. Pray for Steve!" Trump tweeted.
The FBI said it had recovered and was attempting to trace two weapons, a rifle and a handgun, and was actively investigating Hodgkinson's "associates, whereabouts, social media impressions, and potential motivations."
According to his Facebook page, Hodgkinson was a fervent supporter of Sanders, the feisty independent who battled Hillary Clinton for the Democratic presidential nomination last year.
The gunman's social media posts displayed strong anti-Trump sentiment.
"I know he wasn't happy with the way things were going, the election results and stuff," his brother, Michael Hodgkinson, told The New York Times.
Sanders swiftly responded to reports the suspect had volunteered on his presidential campaign.
"I am sickened by this despicable act," he said.
"Let me be as clear as I can be. Violence of any kind is unacceptable in our society and I condemn this action in the strongest possible terms."