A US Capitol police officer, Brian Sicknick, has died of injuries suffered when supporters of President Donald Trump assaulted the legislative building, the force said, bringing to five the number dead from the riot.

Wednesday’s breach of the US Capitol building took place as lawmakers were in the building certifying the victory of President-elect Joe Biden.

“Officer Sicknick was responding to the riots … and was injured while physically engaging with protesters,” the police said in a statement.

He died on Thursday after being taken to hospital following his collapse when he returned to his divisional office, it said. An affiliate of CBS News reported that Sicknick was a 15-year veteran of the force and 40 years old.

The report said he had earlier suffered a stroke and was on life support before his death.

Metropolitan homicide officials will investigate the death of Sicknick, who joined the US Capitol Police in 2008, along with the Capitol force and its federal partners, the police said.

Separately, on Thursday, US Capitol Police chief Steven Sund said in a resignation letter that he would step down effective January 16.

Sund’s resignation was sought by House of Representatives speaker Nancy Pelosi after the federal force charged with protecting Congress was unable to keep Trump’s supporters from storming the Capitol building on Wednesday.

Trump, who initially praised his supporters, later condemned the violence, saying the rioters had defiled the seat of American democracy and must be held accountable. A woman demonstrator was fatally shot by authorities, and three people died from medical emergencies.

After a rally where Trump exhorted his supporters to fight to overturn the November 3 election he lost, hundreds of them stormed the Capitol building on Wednesday, forcing senators and members of the House of Representatives to evacuate as they smashed windows and looted.

US transportation secretary Elaine Chao and education secretary Betsy DeVos resigned on Thursday, joining a growing list of aides leaving Trump’s administration in protest at Wednesday’s events.

As Democratic leaders demanded his ouster, Trump came closer to a formal concession, acknowledging in a video released on Thursday evening that a new administration would be sworn in on January 20 and vowing to ensure a smooth transition.