(CNN) Prosecutors in the US Attorney's office plan to open a federal murder investigation into the death of Brian D. Sicknick, a US Capitol Police officer who died Thursday night, a law enforcement official tells CNN.
Sicknick was injured Wednesday when a mob of President Donald Trump's supporters stormed the US Capitol. He died at approximately 9:30 p.m. ET Thursday "due to injuries sustained while on-duty," Capitol Police officials said in a statement.
The death is being investigated by the DC Metropolitan Police Department's homicide branch, the US Capitol Police and their federal partners.
"Officer Sicknick was responding to the riots on Wednesday, January 6, 2021, at the U.S. Capitol and was injured while physically engaging with protesters. He returned to his division office and collapsed. He was taken to a local hospital where he succumbed to his injuries," the statement read.
Sicknick had joined the Capitol Police in July 2008, and most recently served in the department's First Responders Unit.
"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," Capitol Police said.
Prior to serving with the Capitol Police, Sicknick, who is from New Jersey, was a staff sergeant with the New Jersey Air National Guard. He also served in Operation Enduring Freedom.
"Officer Sicknick gave his life protecting the United States Capitol, and by extension, our very democracy, from violent insurrection. 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," New Jersey Gov. Phil Murphy said in a statement Friday.
House Speaker Nancy Pelosi ordered the flags at the Capitol Building to be lowered to half-staff in honor of Sicknick. The flag atop the White House had not been lowered as of mid-morning Friday, but the White House issued a statement about his death.
"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. 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," deputy press secretary Judd Deere said.
Vice President Mike Pence called Sicknick's family to express his condolences on Friday afternoon, a source familiar with the call says.
Sicknick is the fifth person to die as a result of Wednesday's insurrection. One woman was shot and killed by Capitol Police as the crowd breached the building and three others suffered medical emergencies that proved fatal.
Shortly after 1 p.m. ET Wednesday hundreds of pro-Trump protesters pushed through barriers set up along the perimeter of the Capitol, where they tussled with officers in full riot gear, some calling the officers "traitors" for doing their jobs.
About 90 minutes later, police said demonstrators got into the building and the doors to the House and Senate were being locked. Shortly after, the House floor was evacuated by police. Vice President Mike Pence was also evacuated from the chamber, where he was to perform his role in the counting of electoral votes.
An armed standoff took place at the House front door at about 3 p.m. ET, and police officers had their guns drawn at someone who was trying to breach it.
As a result of the episode, US Capitol Police Chief Steven Sund is resigning. He said in a statement earlier Thursday that Capitol Police and other law enforcement officers were "actively attacked" with metal pipes and other weapons.
"They were determined to enter into the Capitol Building by causing great damage," Sund said.
"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.," he added. "Maintaining public safety in an open environment -- specifically for First Amendment activities -- has long been a challenge."
Still, lawmakers say they are perplexed at the lack of preparedness among law enforcement given that it had been known for weeks that Trump was promoting a rally he said was aimed at preventing the certification of Biden's win.
Rep. Mike Quigley, an Illinois Democrat who was locked in the House chamber during an armed standoff between Capitol Police and a rioter, praised the officers who were in the building that put their lives on the line, but made clear that they were outnumbered and law enforcement was under prepared.
"The Capitol Police I was around did an amazing job under difficult circumstances," Quigley told CNN. "My concern wasn't with how valiant the Capitol Police were. It was that an hour before the debate started, I looked at the throngs of people surrounding different sections of the Capitol and said, we don't have enough security."
Quigley added: "I'm no expert in security, but you can tell we were out-manned in an hour before the debate," referring to Congress' proceedings to certify Biden's win.