The defendants, 22-year-old Fedo Manning, 21-year-old Ronnel Williams Jr., and 19-year-old Chaelyn Groves are not accused of opening fire at the parade ... but instead, prosecutors believe they were part of "illegal firearms trafficking and straw purchases of firearms."

During their investigation into the shooting, police recovered multiple firearms at the scene ... at least two of which they were able to trace back to the alleged illegal gun ring.

One of the weapons, an Anderson Manufacturing AM-15 .223-caliber pistol, was allegedly bought by Manning from a gun store in 2022. He's accused of illegally trafficking dozens of guns.

The second weapon was a Stag Arms 300-caliber pistol was allegedly purchased by Williams for Groves at a gun show in 2023 ... as the latter was too young at the time.

In total, Manning is facing 12 federal charges, while Williams Jr. and Groves were both hit with 4 charges.

None of the men are charged with shooting anyone ... though officials aren't ruling out the possibility that the illegal guns were fired.

In fact, according to court docs, 12 people brandished firearms, and at least 6 fired their weapons during the parade.

"Stopping straw buyers and preventing illegal firearms trafficking is our first line of defense against gun violence," U.S. Attorney Teresa Moore said.

"At least two of the firearms recovered from the scene of the mass shooting at Union Station were illegally purchased or trafficked. We work diligently with our law enforcement partners to keep firearms out of the hands of criminals by investigating and prosecuting those who illegally buy and sell dangerous weapons."

FYI, a straw gun sale is when someone purchases a weapon for another individual.