Lawmakers from both parties Thursday said outrage over the massive security breach at the U.S. Capitol on Wednesday has only grown, and they pledged to determine why the U.S. Capitol Police force was so ill-prepared for the rioters who stormed the complex.

“I’m livid about the whole thing,” Rep. Tim Ryan, D-Ohio, who is leading the House Appropriations Committee investigation into the episode, told reporters. “We were told no one was going to be anywhere close to the Capitol, and the protests. And next thing you know, you turn on the TV, and they’re swinging from the Capitol building with flags.”

Resignations from senior police officials came swiftly — including the resignations Thursday of U.S. Capitol Police Chief Steven Sund, House Sergeant-at-Arms Paul Irving and Senate Sergeant-at-Arms Michael Stenger.

But the departures do not answer painful questions about why Capitol security was so easily overwhelmed, why the Capitol Police failed to request sufficient reinforcements from the D.C. Police and the National Guard, and why some officers appeared to be appeasing the extremist interlopers — with at least one officer allowing a rioter to snap him in a selfie — lawmakers said.

Asked about the disparity between how police responded Wednesday compared to the massive militarized force used during peaceful Black Lives Matter protests over the summer, Rep. Zoe Lofgren, D-Calif., said that was a significant concern.

“This was a group of almost entirely white protesters — rioters, really, not protesters,” she said. “The treatment of them was quite different than had this been a group that was more diverse. So that’s a real issue.”

In a statement issued before the annoucement of his resignation, Sund said that police “had a robust plan established to address anticipated First Amendment activities,” but that the violent attack was “unlike any I have ever experienced in my 30 years in law enforcement.” The statement appeared to suggest the massive size of the assault was not anticipated.

Multiple lawmakers, however, told ABC News that they had raised security concerns in advance of the Jan. 6 joint session of Congress, and they said Sund repeatedly reassured them that the complex would be heavily fortified.

U.S. Rep. Maxine Waters said she spent an hour on the phone with Sund several days before the session, and specifically raised concerns about the potential for violence from the extremists being urged to protest by President Donald Trump. She said Sund promised her the police were ready.

“I thought, he’s the police chief. I guess he knows what he’s doing,” Waters told ABC News. “He’s telling me that they are very comfortable and they have it under control. That the Capitol is going to be secured.”

Anger over the unprecedented breach spanned the congressional ranks. Senate Majority Leader Mitch McConnell called the riot a “massive failure” of the institutions, protocols and planning that are supposed to protect the legislative branch of the federal government.

“A painstaking investigation and thorough review must now take place and significant changes must follow,” McConnell said in a statement. “Initial bipartisan discussions have already begun among committees of oversight and congressional leadership.”

Dismay about the way events unfolded was shared by the officers who had limited resources and reinforcements when they were confronted by an overwhelming mob. One source familiar with the Capitol Police’s security preparations expressed frustration that, even though law enforcement and security officials suspected for days that protesters might try to break into the Capitol, leadership did not request enough assistance from the local Metropolitan Police Department or other law enforcement agencies to defend against such an effort.

Capitol Police Union Chairman Gus Papathanasiou released a statement Thursday saying officers are “frustrated and demoralized by the lack of leadership” that resulted in the storming of the Capitol.

“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 the statement.

Ryan said lawmakers will consult the military and law enforcement agencies involved to perform a “minute-by-minute” review of what went wrong — and that hearings are expected soon.

“We’re never going to look at the Capitol the same way, just like after 9/11, we never looked at terrorism the same way,” Ryan said.

ABC News’ Jack Date contributed to this report.