The FBI will join a homicide investigation into the death of a police officer from injuries sustained while defending the US Capitol from a mob of President Donald Trump’s loyalists.

Brian Sicknick, an Iraq War veteran, died in hospital on Thursday night local time after “physically engaging” with pro-Trump rioters who stormed the complex on Wednesday (local time) to disrupt the certification of Democrat Joe Biden’s presidential election win, according to the Capitol Police.

His death will be investigated by the Washington Metropolitan Police Department’s Homicide Branch, the Capitol Police said in a statement.

The FBI will help in the investigation, Acting Attorney-General, Jeffrey Rosen, added.

“Our thoughts and prayers are with the family and fellow officers of US Capitol Police Officer Brian D Sicknick, who succumbed last night to the injuries he suffered defending the US Capitol, against the violent mob who stormed it on January 6th,” Mr Rosen said in a statement.

Mr Rosen did not say whether the FBI would investigate the death as a homicide, but says the Justice Department “will spare no resources in investigating and holding accountable those responsible”.

During the struggle at the Capitol, Mr Sicknick, 42, was hit in the head with a fire extinguisher, according to two law enforcement officials who could not discuss the ongoing investigation publicly and spoke on the condition of anonymity.

“He returned to his division office and collapsed,” the Capitol Police statement said.

“He was taken to a local hospital, where he succumbed to his injuries.”

Craig Sicknick, the fallen officer’s brother, told the Daily Beast his brother graduated as a Capitol Police officer two days before the 2008 inauguration of Democratic President Barack Obama and he “always tried to do what was right”.

“He worked a lot of overtime, and he was on during this mess,” he added.

The riot forced the resignations of three top Capitol security officials over the failure to stop the breach. Photo: AP

In a statement, Mr Sicknick’s family asked for his death not be politicised.

“Many details regarding Wednesday’s events and the direct causes of Brian’s injuries remain unknown, and our family asks the public and the press to respect our wishes in not making Brian’s passing a political issue,” the family said.

‘So many mourn with and pray for them’

Mr Sicknick’s death brings to five the number of people who died due to the riot.

One woman was shot by a law enforcement officer inside the Capitol building as she tried to break into the House of Representatives chamber.

Three others died of “medical emergencies,” according to police.

Tweet from @guygruber

Mr Sicknick’s death was the fifth linked to the riots, which have led to calls for Mr Trump to be removed from office.

Air Force veteran Ashli Babbitt was shot by a police officer, while three others died after suffering unspecified medical emergencies.

The deaths raise the stakes for Mr Trump, who many Democrats — and some of his fellow Republicans — have blamed for inciting supporters to storm the Capitol in a speech to them on Wednesday morning and after claiming falsely for weeks that the November election had been stolen from him.

Nancy Pelosi, the Democratic Speaker of the House, ordered flags at the Capitol be lowered to half-mast on Friday in Mr Sicknick’s honour.

“The sacrifice of officer Sicknick reminds us of our obligation to those we serve: to protect our country from all threats, foreign and domestic,” Ms Pelosi said in a statement.

“May it be a comfort to officer Sicknick’s family that so many mourn with and pray for them at this sad time.”

The riot that shocked the world and left the country on edge has forced the resignations of three top Capitol security officials over the failure to stop the breach.

Politicians have since demanded a review of operations and an FBI briefing over what they called a “terrorist attack”.

-with agencies