The Maricopa County prosecutor made the assertion in court Wednesday, claiming federal agents told her more than 120 guns and 250,000 rounds of ammunition were discovered in Kelly's home after his arrest.

She said scopes, body armor and silencers were also discovered in the home by authorities during the search ... along with a machine gun in his car.

Kelly has also been accused of hanging bags of white powder from political signs lined with razor blades in Ahwatukee ... a neighborhood south of Phoenix.

Based on the items recovered in the seizure, Bhatia noted agents suspect Kelly "was preparing to commit an act of mass casualty."

According to police, Kelly first fired BB pellets at the campaign office  -- which also serves as Tempe's Democratic National Committee office. He then allegedly fired gunshots on the front door and window of the office.

Bhatia says the shootings took place in the early hours of Sept. 16, Sept. 23, and Oct. 6, and claims Kelly "had guns consistent with the guns that were used" in the shootings.

Thankfully, no one was inside the office building during the shootings ... but a Tempe Police Department spokesperson noted the repeated incidents raised safety concerns for the staffers inside.

Kelly -- who had previously been arrested in 2022 for allegedly stealing campaign signs -- has been charged with multiple felonies, including committing an act of terrorism, unlawful discharge of a firearm, and shooting at a non-residential structure. He's also been charged with misdemeanor criminal damage.