The feds filed legal docs detailing why Alexander Hillel Treisman -- who was originally arrested back in May on child pornography charges -- should remain locked up pending trial. Among the reasons the FBI wants to keep him behind bars ... a possible plot to assassinate the Democratic Presidential nominee.

The feds claim Treisman posted a meme on social media -- asking if he should kill Biden. The feds say he had planned on traveling to a Wendy's that's 4 miles from Biden's home. The feds claim they also found a checklist that ended with the word "execute."

And, then there's this ... in docs, obtained by TMZ, the feds say back in May they found Treisman's van filled with guns and explosives ... from an AR-15 style rifle and a Taurus .380 caliber handgun to a canister of the explosive material Tannerite. What's more ... the feds say Treisman had $509k in U.S. currency, books about survival and bomb-making and drawings of swastikas and planes crashing into buildings.

The feds also say they uncovered driver's licenses from 3 different states. According to docs ... Treisman's Internet searches between March and May 2020 also included information about Biden's home address and night-vision goggles, among other things.

And, if that's not terrifying enough ... the feds allege they found an October 2019 note created on Treisman's Samsung cell phone about a plan to carry out a mass shooting at a mall food court on Christmas Day or Black Friday.