Brian D Sicknick was hit on the head with a fire extinguisher by rioters, eventually dying of his injuries on Thursday night. 

US prosecutors have opened a murder investigation into the death of a US Capitol Police man who died of the injuries he sustained following Wednesday’s rioting at Capitol Hill.

Sicknick, a first responder and member of the Capitol force since 2008, was the fifth person to die following Wednesday’s riot.

The police man’s social media postings have shown he too was a Trump supporter, but the mob attacking him showed no mercy in inflicting fatal injuries on the officer.

Although no footage of the attack has been found, it is known that Sicknick returned to his division’s office after being injured, only to collapse and be taken to hospital where he was later pronounced dead.

Hundreds of his colleagues lined DC’s streets as his body was transported to a morgue last night, with Speaker Nancy Pelosi ordering flags to be lowered to half-mast in his honour.

It remains unclear if any suspects have been identified, or if CCTV footage has been found by investigators, with police only confirming that the force’s homicide division was now investigating the attack.

Sicknick’s brother confirmed his death yesterday, telling ABC7 that he was “a hero”.

“After a day of fighting for his life, he passed away a hero. I would like to thank all of his brothers and sisters in law enforcement for the incredible compassion and support they have shown my family.

“My family and I hope that our privacy can be respected as we grieve. Thank you,” he finished.

Police chiefs have claimed the National Guard were not present in large numbers during or after the initial Wednesday rally because they did not want to incite any rioting, but that decision to under-resource police officers has now proved costly, not least to Capitol Police Chief Steven Sund who resigned yesterday amid growing criticism.

Sergeant at Arms Paul Irving, who is tasked with protecting the House, and Senate Sergeant at Arms Michael Stenger both resigned yesterday as well.

Over 50 police officers were injured in clashes with demonstrators, with two male and two female Trump supporters dying at the riot, one of whom, Ashli Babbit (35), was shot when climbing through a glass door within Capitol buildings.