A police officer has died of injuries sustained during a riot at the US Capitol led by supporters of President Donald Trump, the Capitol Police force said in a statement.
Supporters of Mr Trump stormed the US Capitol on Wednesday as President-elect Joe Biden's election victory was being certified by Congress.
According to the Capitol Police, Officer Brian Sicknick "was responding to the riots...and was injured while physically engaging with protesters".
He died yesterday after being taken to hospital following his collapse upon returning to his divisional office, they added.
He is the fifth person to die following the riot, including one woman shot by police and three others who died of medical emergencies.
Metropolitan homicide officials will investigate the death of Officer Sicknick, who joined the US Capitol Police in 2008, along with the USCP and its federal partners, police said.
Meanwhile, the chief of the Capitol Police is to resign, according to media reports, after the federal force was unable to keep the crowd from storming the building.
House of Representatives Speaker Nancy Pelosi called on Capitol Police Chief Steven Sund to step down after officers in the 2,000-member force fell back as crowds advanced.
That enabled the crowds of Trump supporters to invade the halls of Congress. Other officers fought to keep politicians and staff safe.
The Capitol Police did not immediately respond to inquiries about the reports of Mr Sund's resignation.
Washington's federal prosecutor said he would charge any Capitol Police officers found to be complicit in allowing protesters into the building, and politicians vowed to open an investigation into the department.
"Many of our Capitol Police just acted so bravely and with such concern for the staff, the members, for the Capitol ... and they deserve our gratitude. But there was a failure at the top of the Capitol Police," Ms Pelosi said, in calling for Mr Sund's resignation.
She said that Paul Irving, the House sergeant at arms, would resign. Top Senate Democrat Chuck Schumer said he would fire Michael Stenger, the Senate sergeant at arms, as soon as run-off elections in Georgia for two seats are certified and Democrats control the Senate.
The supporters of Mr Trump ransacked legislators' offices, stole computers and documents, and left threatening messages as they roamed the building for hours.
The crowd racing through the building - with Trump flags and his signature red hats - stood in sharp contrast to the response to anti-racism protests this summer.
At that time, the White House was surrounded by multiple blocks of buffer, and law enforcement officers used tear gas, projectiles and at one point the downward blast of a helicopter rotor to push back protesters.
In comparison, the streets around the Capitol were open on Wednesday morning, and scatterings of Capitol Police stood at low metal barriers resembling bicycle racks.
District of Columbia Councilman Charles Allen, who represents the area around Capitol Hill, called that contrast particularly jarring.
"They were overrun and, in many cases, appear to have completely opened the gates, snapped selfies," Mr Allen said. "By the time they called DC Metropolitan Police Department, it was too late."
A House Democratic aide, speaking on condition of anonymity, said the investigation would focus on what intelligence was available in the run-up to the riot, how decisions were made and why the perimeter of the Capitol was not more secure.
The aide said some protesters were found carrying zip ties. Those heavy-duty plastic ties are often used to handcuff people, and could indicate an intent to kidnap members of Congress.
"Taxpayers deserve to know why ... it took only an hour for a handful of ragamuffin protesters to enter the building and pose a threat to the continuity of democracy," Senator Chris Murphy, the top Democrat on the Senate subcommittee overseeing the Capitol Police, told reporters.
Republican Senate leader Mitch McConnell called the breach a "shocking failure."
The acting US attorney for Washington, Michael Sherwin, told a news briefing that any Capitol Police officer found complicit in the breach of the building would be charged.
Mr Sherwin declined to answer a question about why Capitol Police did not detain people as soon as they attempted to get into the building. "That's a concern and that's made our job difficult," he said.
The FBI sought the public's help in identifying the rioters. Some of the 68 people arrested after Wednesday's incidents were due in court yesterday.
Prior to reports of his resignation, Mr Sund in a statement said the officer who fatally shot Ashli Babbitt, who was among a group of Trump supporters trying to force their way into the House chamber, was on administrative leave while the shooting is investigated.
Mr Sund said his officers "responded valiantly" when demonstrators attacked them with "metal pipes, discharged chemical irritants and took up other weapons." They also faced two pipe bombs.
A senior administration official said the Capitol Police had rejected Defense Department offers of additional training from before Wednesday.