According to the Riverside County sheriff’s office, 49-year-old Vem Miller was stopped at a checkpoint Saturday night not far from Trump's Coachella Valley rally.

Cops claim Miller was illegally in posession of a shotgun, a loaded handgun, and a high-capacity magazine. They say they took him into custody without incident.

There's some confusion over the Miller's alleged motivations ... but, Riverside County Sheriff Chad Bianco claims man presented fake VIP passes and press credentials to officers which led to the search of his vehicle.

Bianco also told the Riverside Press-Enterprise he believes his guys thwarted a third assassination attempt of the former prez ... though he just walked those comments back in a press conference minutes ago.

Now, Bianco says his guys prepped well ... and, he feels that prep stopped any sort of shooting from happening. The United States Attorney's Office says they're aware of the arrest, but it did not affect their protective measures or Trump's safety. Their investigation is ongoing.

Of course, Thomas Matthew Crooks was the first person to try to assassinate former President Trump ... clipping his ear in Butler, Pennsylvania back in July before Secret Service snipers took him down.

Then, just last month, a man named Ryan Wesley Routh allegedly stuck a barrel through the fence at a golf course Trump was at ... but, Secret Service was able to fend off the attack.