U.S. Capitol Police Chief Tom Manger on Sunday said the agency has addressed and fixed several issues in the year following the Jan. 6 Capitol riot, but the one "critical" issue that has yet to be resolved is the staffing shortage it still faces after a surge of retirements and resignations in the wake of the breach.

Manger, who assumed command of the agency in July, appeared on "Fox News Sunday" to address the changes that have been made to security on Capitol Hill since Jan. 6, and what challenges remain.

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"The one thing that we have not been able to fix, so to speak, are the staffing issues and we've lost over 130 officers that have left through either retirements or resignations after January 6," Manger said.

Pro-Trump protesters and police clash at the Capitol on Jan. 6, 2021. (Lev Radin/Pacific Press/LightRocket via Getty Images)

Pro-Trump protesters and police clash at the Capitol on Jan. 6, 2021. (Lev Radin/Pacific Press/LightRocket via Getty Images)

The chief also said that the COVID-19 pandemic also shut down the National Federal Law Enforcement Training Academy for 10 months in 2020, essentially putting a pause on new recruits.

"Between not being able to put any academy classes through the prior year, with the attrition, the way it's been over the past year, we're now really about 400 officers short of where we need to be and that's a pretty critical issue for us," Manger said.

Manger said that the agency has had to shift resources in order to address the increasing number of threats made against Congress in recent years.

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Despite the remaining challenges, Manger said the agency now has the authority to request resources, such as the D.C. National Guard and area law enforcement agencies – improvements that the chief says could help ensure that history doesn’t repeat itself.

"The fact that we have the authority to call out the National Guard, the fact that we have formal processes in place to get additional resources from area law enforcement agencies, is a big improvement, and we believe it would have prevented something like January 6 from happening," he said.