How were U.S. Capitol Police overrun by pro-Trump rioters?

Shock and outrage are abundant on Capitol Hill as major questions arise focused on why U.S. Capitol Police couldn't stop pro-Trump rioters from breaching the Capitol on Wednesday.

The House Appropriations Committee has launched a probe looking into what they call "systemic failure in securing the building's perimeter and in the response once the building was breached."

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Individually, Capitol Police officers who fought off the rioters are being called heroes, but that did not extend to the Chief of Police Steven Sund, who resigned on Thursday.

Law enforcement sources say there are major questions about why there weren't more barriers and more arrests. Former Secret Service Agent Jeff James tells FOX 5 he wants to know why there weren't more officers in place in advance.

"I think that the problem that you had was Capitol Police, much like the Secret Service where I served, is a small agency, and I've mentioned that before," James said. "Even if it had been a situation where it was all hands on deck, they still wouldn’t have had a number matching the mob that there."

READ MORE: Capitol Police rejected federal help to quell pro-Trump mob

Former Police Chief Sund said in a statement Thursday that police were conducting a thorough review of the incident, security planning, policies and procedures. However, Virginia Senator Time Kaine told FOX 5 that's not good enough and says Capitol Police officers are telling him they were not put in a position to effectively deal with Wednesday's mob scene.

"They are extremely unhappy," Sen. Kaine said. "What this officer told me is, ‘Look I don’t know whether we asked for resources and didn’t get them, or we didn’t ask for resources.’ And frankly, I don’t know either so, I haven’t thought about the way to do it." 

U.S. Capitol Police are not releasing the identity of the officer who shot and killed Ashli Babbitt, who had broken into the House of Representatives. However, that officer has been placed on administrative leave and their police powers have been suspended.