What to know about Capitol Police officer Brian Sicknick, who died from injuries after pro-Trump riot

A U.S. Capitol Police officer died Thursday after he was injured in a pro-Trump riot, the fifth person to die in relation to Wednesday's attack on the Capitol building. 

Three people died from medical emergencies on Capitol grounds Wednesday, and one woman was fatally shot by a Capitol Police officer. 

Thursday evening, Capitol Police officer Brian D. Sicknick died from injuries. He joined the force in July 2008 and was part of the department's First Responders Unit, officials said.

Flags were at half-staff in front of the Capitol building Friday morning. Sicknick's death is being investigated as a homicide by federal and local authorities – a development that raises the stakes of the investigation into possible crimes committed during the violent security breach.

Live updates Friday:Capitol rioters are being identified

Here's what we know about Sicknick:

Who was the Capitol police officer?

Brian D. Sicknick, 42, the youngest of three sons, was from South River, New Jersey. He graduated in 1997 from Middlesex County Technical Vocational High School and joined the New Jersey Air National Guard that year.

Sicknick deployed to Saudi Arabia in 1999 in support of Operation Southern Watch. After the 9/11 terrorist attacks, he served in Kyrgyzstan in support of the war in Afghanistan. While stateside, Sicknick served in the 108th Air Refueling Wing out of Joint Base McGuire-Dix-Lakehurst in New Jersey, his brother, Ken Sicknick, said.

He was honorably discharged in 2003, according to Lt. Col. Barbara Brown, a spokeswoman for the New Jersey National Guard.

Sicknick "wanted to be a police officer his entire life," Ken Sicknick said. He "served his country honorably" and made his family "very proud," Sicknicksaid. "Brian is a hero and that is what we would like people to remember."

Public records indicate Sicknick currently lived in Springfield, Virginia.

What happened to the officer Brian Sicknick?

Sicknick died "due to injuries sustained while on duty," U.S. Capitol Police said in a statement. On Wednesday, he "was injured while physically engaging with protesters," police said. He returned to his division office and collapsed, then was taken to a local hospital where he died around 9:30 p.m. Thursday.

Sicknick was hit in the head with a fire extinguisher, according to two law enforcement officials who spoke to the Associated Press.

"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 department said in a statement.

Will there be charges?

U.S. Capitol Police said Sicknick's death will be investigated by the homicide branch of the Washington, D.C., Metropolitan Police Department. Any criminal charges related to Sicknick's death will be federal because the events leading up to it happened on federal property, an official with knowledge of the matter said.

U.S. Reps. Rosa DeLauro, D-Conn., and Tim Ryan, D-Ohio, released a statement late Thursday calling for the "mob who attacked the People’s House" to be held accountable.

"Our hearts break over the senseless death of United States Capitol Police Officer Brian D. Sicknick, who was injured in the line of duty during yesterday’s violent assault on the Capitol," they said in a statement. "Our prayers are with his family, friends, and colleagues on the force."

They added: “This tragic loss should remind all of us of the bravery of the law enforcement officers who protected us, our colleagues, congressional staff, the press corps, and other essential workers yesterday."

The chaos has already led to at least 55 criminal cases filed by the Justice Department against rioters who were charged with unlawful entry, gun violations, theft, assault and others. One man was arrested after officers found a military-style semi-automatic rifle and 11 Molotov cocktails in his possession, said acting U.S. Attorney Michael Sherwin.

More charges are expected in the coming weeks. Sherwin also made clear that no charges, including sedition, rioting and insurrection, are off the table. 

Meanwhile, Capitol Police Chief Steven Sund will resign later this month. The top law enforcement officials in charge of protect the House and Senate have also resigned.

More on the Capitol riots:

Capitol mob drew Trump supporters of all stripes, from a fireman to a 'QAnon shaman'

Woman killed during Capitol riot was a military veteran and staunch Trump supporter

An 'angry' Pence navigates the fallout of his rupture with Trump over election, Capitol riots

Contributing: Kristine Phillips, Kevin Johnson, Cara Richardson and Trevor Hughes, USA TODAY; Michael L. Diamond and Susan Loyer, MyCentralJersey.com