"Quis custodiet ipsos custodes?" That’s Latin for "who watches the watchers?"

It’s a key question as lawmakers try to understand what went wrong protecting the Capitol on Jan. 6 from one of the most malevolent insurrections ever mounted against the American government.

Why was intelligence gathering — and sharing — so deficient leading up to the riot? Why weren’t Capitol Police better prepared?

FBI DIRECTOR WRAY TESTIFIES ON CAPITOL RIOT

And better yet, who makes sure the Capitol Police are up to the job?

"Quis custodiet ipsos custodes?"

Ultimately, the job comes down to the lawmakers themselves. House and Senate Committees have oversight of the sergeants at arms and Capitol Police. Appropriations panels doled out a half billion dollars in money last fiscal year to the USCP. And yet, they got overrun on Jan. 6.

So what does it take for lawmakers to conduct better supervision of the very agency charged with protecting the legislative branch?

"I think that [Congressional leaders] have some responsibility. But they may not have culpability," said government transparency advocate Daniel Schuman. "Congress is optimized to be responsive to people back in the district. They’re going to focus on things that have to do with jobs. The economy. The environment. Those types of things. They’re not optimized to deal with matters regarding the Capitol complex."

And if no one is watching, how can we truly know just how ready are the Capitol Police?

Here’s how one longtime Capitol Hill hand put it to Fox News: "The problem with the Capitol Police is that they think they are going to be defending democracy and they spend all day going through women’s purses."

CAPITOL POLICE CHIEF: INTELLIGENCE SUGGESTS MILITIAS AIM TO 'BLOW UP' BUILDING WHEN BIDEN ADDRESSES CONGRESS 

There are questions as to whether USCP officers thought they had authorization to use lethal force – to protect themselves and others. Was that reminder only given to the Containment and Emergency Response Team (CERT) that day?

What is still stunning is that USCP officers still weren’t certain about rules of engagement for lethal force. Fox News is told officers are reminded regularly to be "courteous," lest they come across a bent-out-of-shape aide or member.

But when it comes time to storm the Capitol on a day Congress is certifying the Electoral College results of a highly contested election …

What happened with intelligence gathering prior to the riot? Why did the Secret Service think it was safe enough to bring Vice President Pence to the Capitol that day? And could the Capitol Police have been better prepared? And, are they ready to deal with another attack?

"I think there are a lot more questions that need to be answered," said Sen. Tammy Duckworth, D-Ill., after a Senate-wide briefing on Capitol security last week.

Capitol Hill residents briefly got their hopes up Monday as crews began working on the outer fence that has hermetically sealed Congress off from the rest of the world since days after the Jan. 6 riot. But an aide to Del. Eleanor Holmes Norton, D-D.C., said the concertina wire-festooned barricade wasn’t coming down at all. Officials were just sliding the fence closer to the Capitol to allow for movement on 3rd Street on the west side of the Capitol. Workers would reinstall the "Green Mile"-style razor wire once they moved the fence.

Not that anyone knew this was the plan. And no one knows exactly what the threat is on Capitol Hill. This is what is so maddening to lawmakers. And, in many respects, terrifying.

After the security briefing, Sen. Ted Cruz, R-Texas, argued that the "barbed wire" made the Capitol resemble "a military installation in Baghdad." Cruz argued there was no need for the National Guard any longer.

"To maintain their deployments for months on end, absent an imminent threat, isn’t justified," crowed Cruz.

Well, there is a threat. Some kind of threat. Depending on who you talk to or what chat room into which you wander.

CAPITOL POLICE UNION SLAMS 'DYSFUNCTION' AT RIOT HEARING AS COPS SUFFER CRUSHED SPINAL DISCS, BRAIN INJURIES

8Kun and other dark websites are humming about March 4. President Biden took office at noon ET on Jan. 20, as prescribed by the 20th Amendment, ratified in 1933.

But QAnon loyalists to former President Trump believe that the 45th commander in chief is slated to rally and return to the scene to be sworn in on March 4. That’s the original day on which the U.S. inaugurated presidents. This alternative mythology is pushed by the sovereign citizen movement. Many don’t recognize American laws nor federal currency. There is also some chatter about March 20, the day the Republican party came to life in 1834. And, there’s even some noise about April 15: federal income tax day.

A memo obtained by Fox News from acting House Sergeant at Arms Timothy Blodgett indicates that some threat for March 4 has diminished.

There is a big argument about how legitimate some of those threats are to the Capitol. Then again, Capitol security officials didn’t see January 6th coming, either. Multiple Congressional security officials contacted by Fox News in the days leading up to the Jan. 6 siege and said their concerns on electoral ballot certification day centered on one thing: "lone wolves." Speaking to Fox News on the condition of anonymity after the attack, one security source wondered after the attack if intelligence was either slanted against presenting officials with an "honest" assessment of the threat facing the Capitol that day. Or, perhaps something just as unsettling.

"Maybe they were just incompetent," said the source.

Does this pertain to U.S. Capitol Police intelligence? Or overall, federal government intelligence, involving the Department of Homeland Security, FBI and Joint Terrorism Task Force (JTTF)?

That’s hard to piece together.

"How could the security planning policies and procedures, apparently be so lack and ill prepared," asked Rep. Tim Ryan, D-Ohio, at a recent hearing on Congressional security. "The event was widely promoted on social media weeks in advance."

At the same hearing, acting USCP Chief Yogananda Pittman testified that her forces weren’t ready for the number of marauders who hit the Capitol that day. She declared that 10,000 protesters showed up on the Capitol campus and 800 stormed the building.

"We know that members of the militia groups that were present on January 6th have stated their desires that they want to blow up the Capitol and kill as many members as possible with a direct nexus to the State of the Union," said Pittman. "We think that it’s prudent that Capitol Police maintain its enhanced and robust security posture until we address those vulnerabilities going forward."

CLICK HERE TO GET THE FOX NEWS APP

But it’s really hard to know what’s the proper posture. Fencing? Permanent fencing? A National Guard contingent? It’s unclear what USCP need or want.

"They are the most difficult agency that I’ve ever encountered in the federal government to try to get any information out of," said Schuman. "When you have a well-funded agency that is completely insulated from any type of oversight, from any type of accountability, where you end up is with a paper tiger."

When they laid the Capitol cornerstone, George Washington said, "It may be relied upon, it is the progress of this building that will inspire or depress the public confidence."

Who watches the watchers?

The debate is over the United State Capitol Police.

And if there’s going to be confidence in congressional security, it has to come from Congress itself.