"You knew you could count on him to do his job and be right there when you needed him," Officer Brian D. Sicknick's colleague tells PEOPLE

A colleague of Brian D. Sicknick is mourning the loss of the United States Capitol Police officer, who died from injuries he sustained during Wednesday's violent riots.

“He was a great guy to work with. You knew you could count on him to do his job and be right there when you needed him. He believed in this country and he believed in all of us [who worked with him],” the colleague, who did not wish to be identified, tells PEOPLE.

Although officials have not detailed Sicknick’s injuries, the colleague — who was not with the late officer at the time — says that he “got hit in the head.” Police said that Sicknick collapsed after returning to his division office and was taken to a hospital, where he later succumbed to his injuries.

"Finally it came down that he passed. It was a hard call to get," the colleague says.

United States Capitol Police Officer Brian D. Sicknick
Officer Brian D. Sicknick
| Credit: USCP

The late officer’s colleague went on to express outrage over the lack of protection officers working that day received.

“It makes me sick in a way I never felt before, over what happened to Brian. He was out there doing his job. He put his heart into it. This was his mission, protecting the Capitol. It makes me sick that he was there on the front lines and no protection. He wasn't at war in some faraway place. He was here at home,” the colleague tells PEOPLE.

Continues the colleague, “He was attacked. He was outnumbered. We all were outnumbered."

Sicknick, 42, died at 9:30 p.m. Thursday from injuries he sustained "while physically engaging with protesters" at the riots, USCP said in a statement.

"The entire USCP Department expresses its deepest sympathies to Officer Sicknick’s family and friends on their loss, and mourns the loss of a friend and colleague," the USCP said in its statement.

pro Trump protesters breach Capitol building
Rioters at the U.S. Capitol building on Wednesday
| Credit: Tasos Katopodis/Getty
Capitol building breached by pro Trump protesters
U.S. Capitol building breached
| Credit: Graeme Sloan/Bloomberg via Getty

Acting Attorney General Jeffrey A. Rosen said on Friday that the case will be investigated by the FBI and the Metropolitan Police Department.

“Our thoughts and prayers are with the family and fellow officers of U.S. Capitol Police Officer Brian D. Sicknick, who succumbed last night to the injuries he suffered defending the U.S. Capitol, against the violent mob who stormed it on January 6th,” Rosen wrote. “ The FBI and Metropolitan Police Department will jointly investigate the case and the Department of Justice will spare no resources in investigating and holding accountable those responsible.”

The officer's death marked the fifth fatality in connection to the riots, in which a large group of pro-Trump supporters stormed the Capitol building as lawmakers gathered to certify Electoral College votes for President-elect Joe Biden.

Ashli Babbitt, 35, was fatally shot by an officer who was confronting rioters as they stormed into the building, according to police. Authorities confirmed on Thursday morning that three other people had died from "medical emergencies." They were identified as Rosanne Boyland, 34, Kevin Greeson, 55, and Benjamin Phillips, 50.

According to the Capitol Police, over 50 officers from the department and Metropolitan Police Department sustained injuries, while several requiring hospitalization.

"The violent attack on the U.S. Capitol was unlike any I have ever experienced in my 30 years in law enforcement here in Washington, D.C.," Capitol Police Chief Steven Sund, who resigned from his post on Thursday, said in a previous statement. "Make no mistake – these mass riots were not First Amendment activities; they were criminal riotous behavior."