Updated at 9 p.m. ET:
A day after an insurrection that overtook the U.S. Capitol, the Capitol's three top security officials resigned from their posts amid building pressure from lawmakers and others over failures that allowed the dramatic breach.
The House and Senate's top protocol officers and the U.S. Capitol Police chief are now all expected to be replaced following a series of resignations in the wake of the security failures.
Senate Majority Leader Mitch McConnell said he accepted the resignation of Senate Sergeant-at-Arms Michael Stenger late Thursday. Earlier Thursday, incoming Senate Majority Leader Chuck Schumer said he would fire Stenger if he didn't quit first.
"Today I requested and received the resignation of Michael Stenger, the Senate Sergeant at Arms and Doorkeeper, effective immediately," McConnel said.
McConnell said lawmakers are beginning to "examine the serious failures that transpired yesterday and continue and strengthen our preparations for a safe and successful inauguration on January 20th."
Earlier Thursday, U.S. Capitol Police Chief Steven Sund resigned his position hours after House Speaker Nancy Pelosi and others called for his removal.
"I am respectfully submitting my letter of resignation, effective Sunday, January 16, 2021," Sund said in his letter Thursday. "It has been a pleasure and true honor to serve the United States Capitol Police Board and the Congressional community alongside the women and men of the United States Capitol Police." Sund earlier had defended his department.
Pelosi told reporters she hadn't even heard from Sund after Wednesday's attack on the Capitol, and made it clear to his office that she would be calling for his ouster.
She also said House Sergeant-at-Arms Paul Irving, the chamber's top protocol officer, who helps direct security plans, would be submitting his resignation as well.
"There was a failure of leadership at the top of the Capitol Police," Pelosi said.
Ahead of Sund's resignation, the U.S. Capitol Police union also issued a statement calling for the removal of agency leadership. Gus Papathanasiou, head of the union, said a lack of planning, backup, equipment and communication failures led to the greatest breach of the Capitol in more than 200 years.
"Without a change at the top, we may see more events unfold like those we saw on January 6th. We cannot leave our officers and the Capitol Hill community they protect, to the mercy of further attacks amid a vacuum of leadership," Papathanasiou said. "Until we have a leadership team at USCP that is willing to work hand in hand with the Union and our Officers as one team, we will continue to have systemic failures."
Schumer, who initially called for the firing of Stenger, had noted that the current majority leader, Sen. Mitch McConnell of Kentucky, appointed Stenger.
"He did a terrible, terrible job," said Schumer — who is due to take over as majority leader after Democrats won both Senate seats in Georgia's runoff elections this week.
While many lawmakers prefaced their concerns by applauding the bravery of the Capitol Police and others, they also said the hours-long attack should never happen again.
Republican Sen. Lindsey Graham of South Carolina had also agreed with Schumer that Stenger should resign. Graham said there was better protection in place during Black Lives Matter marches last year and that Wednesday could have been much worse.
"Anyone in charge of defending the Capitol failed their duties. If they would have been in the military, they would have been relieved of their commands and most likely court-martialed," Graham said. "So the first thing that has to happen is to hold those accountable for failing to defend the nation's Capit0l while the Congress was in session."
Graham also called for a joint task force to identify every individual who breached the Capitol, occupied chambers, destroyed property and entered offices.
In a statement earlier Thursday recounting arrests and some other details from the attack, Sund had defended the agency. He also pointed to challenges in defending an "open environment," especially when it comes to First Amendment activities.
He called the actions of his officers "heroic."
"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.," Sund said. "Maintaining public safety in an open environment — specifically for First Amendment activities — has long been a challenge. The USCP had a robust plan established to address anticipated First Amendment activities."
McConnell earlier on Thursday said while the ultimate blame lies with the "unhinged criminals" who broke into the Capitol, it doesn't preclude Congress from addressing its "shocking failures" in security.
"Yesterday represented a massive failure of institutions, protocols, and planning that are supposed to protect the first branch of our federal government," McConnell said. "A painstaking investigation and thorough review must now take place and significant changes must follow."
McConnell's concerns echoed those heard among members in the aftermath of the insurrection, which paralyzed the nation's Capitol. More than 50 law enforcement officers were injured and four civilians died, including one woman shot by a Capitol Police officer.
The security breakdown came in the form of pro-Trump extremists who stormed the Capitol complex and stopped the counting of electoral votes to confirm President-elect Joe Biden's win. It's now expected to become a centerpiece of debate in the coming weeks, especially ahead of Biden's Jan. 20 inauguration.
"You can be assured that somebody is going to be held responsible for this," Rep. Tim Ryan, D-Ohio, who heads up the appropriations subpanel that oversees Capitol Police funding, told reporters in a call Thursday. "We're going to review what the plans were and what exactly happened, and then I will assure every American that the appropriate people will be held accountable. Because this was an embarrassment, and it was unacceptable."
Ryan joined House Appropriations Committee Chairwoman Rosa DeLauro, D-Conn., in a statement denouncing the failed "coup attempt" instigated, they say, by President Trump. However, Ryan and DeLauro said the Capitol's security breach raises serious questions about what law enforcement did and what should have been done differently.
Both said their committee would "robustly" investigate Wednesday's events, including holding hearings to question leaders directly about what went wrong. Capitol Police, which employs more than 2,000 workers, including police officers and civilians, received $516 million in funding for the current fiscal year.
Ryan told reporters that the entire available Capitol Police force, at an estimated 1,400 to 1,500 officers, plus roughly 1,000 officers from Washington, D.C.'s Metropolitan Police Department, were on the scene at the Capitol at the time of the incursion. He said the extremist mob was violent, swinging lead pipes and other weapons at officers, leaving 15 officers in the hospital, including one in critical condition.
"I'm livid about the whole thing because I had conversations with the sergeant-at-arms and the chief of the Capitol Police and assurance that every precaution was being taken," Ryan said. "We were told nobody was going to be anywhere close to the Capitol."
Ryan said he was told the initial assessment was that law enforcement didn't anticipate violence despite explicit calls for it in the days leading up to the event. He said rank-and-file offers did the best they could in circumstances where they were overrun, without backup and support. Ryan noted that Capitol Police held protesters off for more than hour before the incursion.
"This has kind of broken the veil of protection in the Capitol," Ryan said. "This was basically domestic terrorism at one of the great shrines of our democracy."
Five House committee chairs released a letter Thursday evening to FBI Director Christopher Wray demanding an "immediate and urgent briefing" on the agency's efforts to investigate the attack.
Wray released a statement earlier Thursday saying the FBI had "deployed our full investigative resources" and was coordinating with other law enforcement agencies. "Our agents and analysts have been hard at work through the night gathering evidence, sharing intelligence, and working with federal prosecutors to bring charges." The FBI set up a tip line for information or images of those involved.
House Democratic lawmakers noted Biden's upcoming swearing-in made the probe urgent.
"Given the incendiary environment caused and exacerbated by President Trump's rhetoric, along with the upcoming inauguration of President-Elect Joe Biden, it is imperative that the FBI leverage all available assets and resources to ensure that the perpetrators of this domestic terrorist attack and those who incited and conspired with them are brought to justice, and that this domestic terrorist group is disrupted from further actions against our government," wrote House Oversight Committee Chair Carolyn Maloney, Judiciary Committee Chair Jerry Nadler, Homeland Security Chair Bennie Thompson, Intelligence Chair Adam Schiff, Armed Services Chair Adam Smith and Rep. Stephen Lynch.
NPR producer Barbara Sprunt contributed to this report.