The police officer killed on Capitol Hill by a mob whipped into a violent frenzy by Donald Trump was a fellow supporter of the soon-to-be-ex-president.

Officer Brian Sicknick, 42, was pepper-sprayed and hit in the head as he battled the invaders, his father said.

Ambulance crews twice resuscitated the Gulf War veteran as he was rushed to a nearby Washington hospital but he died the next day.

US Capitol Police officer Brian Sicknick gave his life defending democracy against a raging mob. Photo: AP

“He ended up with a clot on the brain,” his father said. “If they had operated on him, he would’ve become a vegetable.

“He loved his job … I’ll never get over this.”

Mr Sicknick was a Trump supporter, his father said, adding that his son’s political views never interfered with the execution of his duty to protect and serve.

His first big assignment was working at former President Barack Obama’s inauguration in 2009. He had just been sworn in with the US Capitol Police Department at the time, fulfilling a lifelong dream.

His death is being investigated as a homicide by the Washington Metropolitan Police and FBI, which have made a series of arrrests since the melee.

As a member of the department’s First Responder’s Unit, Sicknick was the sixth US Capitol Police officer to die in the line of duty.

Sicknick’s father said Vice-President Mike Pence and House of Representatives Speaker Nancy Pelosi called the family on Friday to offer their condolences.

Brimming with hate and fury, Donald Trump’s supporters storm Capitol Hill. Photo: AP

During the call, Pelosi invited the family to the Capitol to pick out a spot for a plaque that will be made in Sicknick’s honour. Funeral services will also be held at the Capitol, according to his father.

Trump has not publicly commented on the officer’s death or the other four people who died when his supporters overran the Capitol building this week following Trump’s “Save America” rally where the president exhorted his followers to fight.

“If any good comes out of my son’s death, I just hope that it stops all the lunacy that’s been going on in this country,” his father said.

-with AAP