The woman who died Wednesday after participating in the coup in the Congress compound was shot dead by Capitol Hill police, the Washington police chief said.
A staunch supporter of President Donald Trump, her name was Ashli Babbitt and lived in the San Diego area of southern California, US media reported.
She was part of a group of protesters who wreaked havoc on Capitol Hill as elected officials began to certify Democrat Joe Biden's victory.
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Pro-Trump violence on Capitol Hill: "American democracy is on the brink of death"
"Uniformed officers of the Capitol Police confronted them and at one point one of them used his service weapon" and touched her, said at a press conference on chef Robert Contee.
She was pronounced dead after being transferred to hospital.
An internal investigation was opened into this "tragic event", he added.
"Nothing will stop us .."
Former soldier, Ashli Babbitt carried out "four deployments with the American air force", according to the television channel KUSI, which says to have spoken with her husband.
The latter had not accompanied her to Washington to demonstrate in support of the Republican president, who refuses to concede defeat.
"I really don't know why she decided" to intrude on the Capitol, said Ashli Babbitt's mother-in-law, quoted by a Fox 5 reporter.
On her Twitter account, she introduced herself as a "veteran" and "libertarian", showing her love for her country.
She had recently retweeted numerous messages from people traveling to Washington to protest at Donald Trump's call.
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On Tuesday, Ashli Babbitt replied to one of them who complained about the cancellation of her flight: "Nothing will stop us ... They can try, try and try but the storm is here and it is coming down on (Washington) DC in less than 24 hours… From black to light!
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Three other people dead around the Capitol
Three other people - a woman and two men - died in the vicinity of the Capitol "of a separate medical emergency," Mr Contee said, without giving further details or saying whether they were participants in the protests.
The causes of their deaths cannot be established until they are examined by a forensic doctor, he added.
The police, according to him, also proceeded to 52 arrests Wednesday, including 26 in the enclosure of the Capitol.
To ward off possible new protests, Washington Mayor Muriel Bowser has extended the state of emergency in the federal capital for 15 days, until the end of Donald Trump's mandate.
This will allow him, if necessary, to renew the curfew decreed for the night from Wednesday to Thursday, or to requisition additional resources.
VIDEO. United States: pro-Trump protesters take over Capitol Hill
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