WASHINGTON (Nexstar) — Flags over the US Capitol flew at half-staff Friday to honor Capitol Police Officer Brian Sicknick, who died Thursday from injuries he suffered in the insurrection Wednesday.

Both the FBI and the Washington Metropolitan Police Department Homicide Unit are investigating his killing. Authorities said Sicknick collapsed after returning to his office after being attacked by the rioters. The 13-year veteran of the force was rushed to a hospital and died the next day.

President-elect Joe Biden said, “Our sympathies and our concerns go out to his family, and the people responsible should be held accountable.”

Biden later said the mob was made up of “terrorists, domestic terrorists. The difference here is this had the active encouragement of a sitting President of the United States.”

Congressman Tim Ryan, a Democrat from Ohio, said more than 50 other officers were also seriously injured and 15 were hospitalized after being attacked by the pro-Trump mob.

“These guys were hitting the Capitol Police in the head with lead pipes,” Ryan said.

Some of those pipes and other weapons were disguised as flag poles. In other cases, fire extinguishers were only carried and used to attack officers and property.

US Capitol Police Chief Steven Sund announced his resignation Thursday. Lawmakers also want investigations into the lack of preparedness and failure in intelligence gathering as many right-wing groups had publicized threats on social media prior to the attack.