By JACK DATE, MARK OSBORNE and EMILY SHAPIRO, ABC News

(WASHINGTON) — The U.S. attorney’s office in D.C. has opened a federal murder investigation into the death of U.S. Capitol Police officer Brian Sicknick, who died Thursday night after suffering injuries in the violent siege at the Capitol, three law enforcement sources confirmed to ABC News.

The investigation is being conducted jointly between the FBI and the D.C. Metropolitan Police Department, with cooperation from U.S. Capitol Police.

Sicknick responded to Wednesday’s riots and “was injured while physically engaging with protesters,” Capitol Police said in a statement. “He returned to his division office and collapsed.”

He was taken to a hospital where he died at 9:30 p.m. Thursday, police said.

Sicknick, 42, had been with the department for 12 years, most recently working as part of the first responder unit.

Sicknick’s death is the fifth connected to Wednesday’s riots, which saw swarms of pro-Donald Trump protesters overwhelm police barricades, surge into the U.S. Capitol and force lawmakers to go into hiding.

House Speaker Nancy Pelosi has ordered the flags at the Capitol to be flown at half-staff in Sicknick’s honor.

“On behalf of the House of Representatives, I send our deepest condolences to the family and loved ones of Officer Brian Sicknick, who died after defending the Capitol complex and protecting those who serve and work here,” Pelosi said in a statement Friday.

“The perpetrators of Officer Sicknick’s death must be brought to justice,” Pelosi said. “The violent and deadly act of insurrection targeting the Capitol, our temple of American Democracy, and its workers was a profound tragedy and stain on our nation’s history. But because of the heroism of our first responders and the determination of the Congress, we were not, and we will never be, diverted from our duty to the Constitution and the American people.”

“The sacrifice of Officer Sicknick reminds us of our obligation to those we serve: to protect our country from all threats foreign and domestic,” Pelosi said. “May it be a comfort to Officer Sicknick’s family that so many mourn with and pray for them at this sad time.”

Sicknick, the youngest of three siblings, had the biggest heart, his family said.

His brother said in a statement to ABC News, “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,” he added.

Virginia Democratic Rep. Don Beyer is calling for Sicknick to lie in state in the Capitol Rotunda.

“He made the ultimate sacrifice while protecting those trapped in the Capitol amid a violent assault on our democracy itself,” Beyer said in a statement Friday. “Like others before him who died in defense of the people’s representatives, he deserves to lie in state.”

The White House’s first comment on Sicknick’s death came Friday morning in a written statement from deputy press secretary Judd Deere.

“Anytime a member of law enforcement dies in the line of duty it is a solemn reminder to us all that they run toward danger to maintain peace,” Deere said. “The President and the entire Administration extend our prayers to Capitol Police Officer Brian Sicknick’s family as we all grieve the loss of this American hero.”

Other lawmakers reacted overnight to Sicknick’s death, including Washington, D.C., Mayor Muriel Bowser, who tweeted: “On behalf of the residents of the District of Columbia, our thoughts & prayers are with the family, friends and colleagues of United States Capitol Police Officer Brian D. Sicknick. May he Rest In Peace, and we work tirelessly to honor his service to the Congress and our nation.”

Sen. Ted Cruz, who has been criticized for his attempt to overturn the 2020 election, wrote on Twitter: “Devastating. Heidi and I are lifting up in prayer the family of the U.S. Capitol Police officer who tragically lost his life keeping us safe. He was a true hero. Yesterday’s terrorist attack was a horrific assault on our democracy. Every terrorist needs to be fully prosecuted.”

Sen. Marsha Blackburn tweeted: “I am truly devastated to learn that a Capitol Police officer has died as a result of yesterday’s violence at the Capitol. My heart goes out to the officer, their family, and all of our front line law enforcement that put their lives on the line each day to protect this nation.”

Ohio Rep. Tim Ryan said his heart is broken over the loss and added that Sicknick’s death should serve as a “reminder of the bravery of the law enforcement who protect us every day.”

Rep. John Curtis, R-Utah, wrote: “I am devastated to hear about the passing of U.S. @CapitolPolice Officer Brian Sicknick and extend my deepest sympathy to his family. My staff and I cannot say enough to express our appreciation for his sacrifice to keep us safe.”

“My sincerest condolences to the Sicknick family,” tweeted Rep. Veronica Escobar. “I’m grateful for Officer Sicknick’s service to our country and his work to protect everyone who worked at the Capitol. My heart goes out to his loved ones.”

Rep. Steve Scalise, R-La., tweeted: “Devastating news. Please join me in praying for our fallen Capitol Police officer’s family during this heartbreaking time.”

Four others died in the Capitol siege. Three people died of medical emergencies, while a 35-year-old woman, Ashli Babbitt, was shot and killed by Capitol Police while trying to enter a broken window into the House Chamber.

Law enforcement said more than 50 Capitol Police and Metropolitan police officers suffered injuries in the rioting and “several USCP officers have been hospitalized with serious injuries.”

“These individuals actively attacked United States Capitol Police Officers and other uniformed law enforcement officers with metal pipes, discharged chemical irritants, and took up other weapons against our officers,” Capitol Police said. “They were determined to enter into the Capitol Building by causing great damage.”

Capitol Police Chief Steven Sund resigned on Thursday afternoon in the wake of the riots. Several prominent people had called for him to step down, including Pelosi and the head of the police union.

Capitol Police Union Chairman Gus Papathanasiou said in a statement Thursday that officers were “frustrated and demoralized by the lack of leadership.”

“We have several protesters dead, multiple officers injured and the symbol of our Democracy, the U.S. Capitol, desecrated. This never should have happened,” Papathanasiou said in a statement.

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