US Capitol police officer Brian Sicknick has died of injuries suffered during a riot led by supporters of President Donald Trump, the force said in a statement late on Thursday.
Wednesday's breach of the building was a stunning assault on American democracy at a time when Congress was certifying the victory of President-elect Joe Biden.
"Officer Sicknick was responding to the riots...and was injured while physically engaging with protesters," police said in a statement.
He succumbed on Thursday after being taken to hospital following his collapse upon returning to his divisional office, they added.
Metropolitan homicide officials will investigate the death of Sicknick, who joined the US Capitol Police in 2008, along with the USCP and its federal partners, police said.
Trump condemned the violence, saying rioters had defiled the seat of American democracy and must be held accountable. Critics and some world leaders have said he is to blame for the attack by his supporters on the US capitol. He had been claiming election fraud without proof.
Among the four more killed were a woman demonstrator shot by authorities.