"We don't have the manpower," said one official told the outlet, noting that many J6 participants who will eventually be charged haven't been arrested yet.

Another official said that the cash crunch was a "culmination" of various factors - including the need to provide support for cases moving to trial.

A third official called it a "work-in-progress" as prosecutors from US attorney's offices around the country are recalled to their offices.

The request comes after federal officials have made around 850 arrests in the nearly 19 months since the incident - just a fraction of the 2,500 people who entered the Capitol, and the "hundreds more who committed serious crimes outside but haven't yet been arrested."

The massive trove of evidence — be it body camera and surveillance video or damning content generated by suspects themselves — presents a tremendous challenge for an enormous bureaucracy working with technology that's often a few years behind the times, at best.

The U.S. Attorney's Office for the District of Columbia, which is overseeing the Capitol siege investigation, is also running separate inquiries exploring the fake electors scheme and a conspiracy to obstruct the Jan. 6 electoral vote certification, both of which touch on Trump's actions in the lead-up to Jan. 6, as well as on the day of the attack. -NBC News