The suspect who authorities say fired on Republican lawmakers as they played baseball raged against Republican US President Donald Trump on social media and idolised Bernie Sanders.
On Wednesday a senior US official named the gunman as James T Hodgkinson from Illinois. Media reports said he was a 66-year-old home inspector.
He died from injuries sustained in a shootout with Capitol Hill police who were at the scene in Alexandria, Virginia.
He is believed by investigators to have been a person "of strong views", the US official said, without elaborating.
House of Representatives Majority Whip Steve Scalise was among those injured in the shooting.
In a dramatic blow-by-blow account, Representative Mo Brooks of Alabama told CNN the gunman was armed with a rifle and appeared to be a white male, "a little bit on the chubby side".
Brooks said he saw the man only for a second, and that he was shooting from a chain link fence behind the third base position on the field where the congressional group was holding its morning practice.
"There must have been 50 to 100 shots fired," he told CNN. "I hear Steve Scalise over near second base scream. He was shot," said Mr Brooks, adding he helped apply a tourniquet with his belt to a congressional staffer who was shot in the leg.
"One of our security detail was shooting back, but it was our pistol versus the shooter's rifle," Brooks said. "The only weapon I had was a baseball bat."
Fox News anchor Bret Baier tweeted that Scalise was shot in the hip and is expected to survive.
US President Donald Trump, a Republican, said he and Vice President Mike Pence were aware of the shooting and are monitoring developments closely.
"We are deeply saddened by this tragedy. Our thoughts and prayers are with the members of Congress, their staffs, Capitol Police, first responders, and all others affected," Mr Trump said in a statement.
Sanders condemns shooting
Mr Sanders said in a statement on Wednesday that the suspected gunman was somebody who had "apparently volunteered" on his presidential campaign. He condemned the shooting, saying he was "sickened by this despicable act".
The Belleville News-Democrat, the local newspaper, posted a photo of Hodgkinson protesting outside a post office there in 2012, wearing sunglasses and a goatee and holding a homemade placard that read "TAX the Rich".
Hodgkinson was a member of many anti-Republican groups on Facebook including 'The Road to Hell Is Paved With Republicans', 'Terminate The Republican Party', and 'Donald Trump is not my President', a search of what appeared to be his profile showed.
Mr Trump won the US Presidential election in November and took office in January. Republicans also control both chambers of Congress.
His timeline was headed by a cover photograph of Mr Sanders, a US senator from Vermont who campaigned unsuccessfully to be the Democratic Party's presidential candidate last year.
A review of his online posts stretching back several years found that his public posts were almost exclusively about politics, and that they often sharply criticised Republican politicians and policies.
Beginning around the summer of 2015, Hodgkinson began often expressing enthusiastic support for Mr Sanders' 2016 campaign.
Mr Sanders, an independent, ran an insurgent campaign during the Democratic primary as a progressive populist but was defeated in the nominating contests by Hillary Clinton.