The U.S. attorney's office in Washington has opened a federal murder investigation into the death of Capitol Police officer Brian Sicknick, who died Thursday night after suffering injuries in the violent siege on Capitol Hill, 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.
Acting Attorney General Jeffrey Rosen said in a statement that the Department of Justice "will spare no resources in investigating and holding accountable those responsible."
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.
According to sources familiar with the matter, authorities believe Sicknick's death was driven by a medical condition. They're also investigating reports that he was attacked with a fire extinguisher or another item at the Capitol, sources said. So far, reports of an attack haven't been confirmed and authorities are hoping to locate video or other imagery from the scene.
Sicknick's family said in a statement, "Many details regarding Wednesday's events and the direct causes of Brian's injuries remain unknown and our family asks the public and the press to respect our wishes in not making Brian's passing a political issue."
Sicknick, a 42-year-old military veteran, had worked for the Capitol Police for 12 years.
Sicknick's family said he "wanted to be a police officer his entire life."
"After a day of fighting for his life," one of his brothers said in a statement, "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."
Sicknick leaves behind his parents, two brothers and his girlfriend of 11 years.
Sicknick's death is the fifth connected to Wednesday's riots, which saw swarms of pro-Donald Trump rioters overwhelm police barricades, surge into the Capitol and force lawmakers to flee or hide.
House Speaker Nancy Pelosi has ordered all 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 added. "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."
"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."
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 comments on Sicknick's death came Friday morning in a 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."
Vice President Mike Pence tweeted Friday afternoon, "Officer Sicknick is an American hero who gave his life defending our Capitol and this Nation will never forget or fail to honor the service and sacrifice of Officer Brian Sicknick."
President-elect Joe Biden on Friday offered his sympathy to Sicknick's family, adding, "The people responsible should be held accountable. And they will be."
In 1997, Sicknick, a New Jersey native, enlisted in New Jersey's Air National Guard.
Sicknick deployed to Saudi Arabia in 1999 and to Kyrgyzstan in 2003, said Lt. Col. Barbara Brown, a spokeswoman for the New Jersey National Guard. He was honorably discharged in 2003.
"Officer Sicknick gave his life protecting the United States Capitol, and by extension, our very democracy, from violent insurrection," Murphy said in a statement. "His needless murder at the hands of a mob bent on overthrowing the Constitution he had dedicated his life to upholding is shocking. It is my fervent hope that the rioters whose actions directly contributed to his death are quickly identified and brought to justice."
Virginia Gov. Ralph Northam added, "Officer Sicknick was killed while doing his job -- defending those trapped in the Capitol building amid a violent attack on our democracy. His death is a tragedy, and those responsible must be prosecuted to the fullest extent of the law."
Federal Law Enforcement Officers Association President Larry Cosme called Sicknick an "American hero."
"Officer Sicknick and his brothers and sisters in the U.S. Capitol Police were the thin blue line protecting not just our elected officials from violent insurrectionists, but the very heart of American democracy," Cosme said in a statement. "Political violence in this country in any form is deplorable. We support a full and vigorous investigation and prosecution of all involved in carrying out this heinous assault."
Cosme added, "We urge the Justice Department to pursue the maximum penalties for the death of this officer and all of the assaults on law enforcement that occurred on that day for anyone responsible for orchestrating, participating, and carrying out Wednesday's invasion of the U.S. Capitol."
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."
Republican Sen. Ted Cruz, who has been criticized for his attempt to overturn 2020 election results, 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."
Rep. Tim Ryan, D-Ohio, 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."
Among those who died in the siege, three people suffered 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 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.
Capitol Police Chief Steven Sund resigned on Thursday afternoon in the wake of the riots. Pelosi was among those calling for his resignation.
Capitol Police Union Chairman Gus Papathanasiou said in a statement Thursday that officers were "frustrated and demoralized by the lack of leadership."
"This never should have happened," Papathanasiou said.
ABC News' Luke Barr, Vera Drymon, Jenn Leong, Mike Levine, Alex Mallin and John Parkinson contributed to this report.