The Secret Service dropped the ball in spectacular fashion at Donald Trump‘s campaign rally where he was nearly killed by a would-be assassin … so says a former USSS special agent.

TMZ Studios has a new special, “TMZ Presents: Under Fire: The Trump Assassination Attempt,” where we explore the many, stunning security failures that lead to the attempt on Trump’s life at a Butler, Pennsylvania rally.

Richard Staropoli, a former Secret Service Special Agent, doesn’t mince words … he says Trump’s security detail totally failed him and left him inexplicably exposed to mortal danger.

Shooter Thomas Matthew Crooks slipping past security and taking perch on a rooftop with an AR-15 style rifle and a clear view of Trump is the epitome of failure by the Secret Service … at least as far as Richard is concerned.

Another former USSS Special Agent, Cheryl Tyler, has a different view. She’s planned security for similar presidential events and says the security plan here was “textbook.”

Still, Richard and Cheryl agree … the building where Crooks squeezed off shots from should have been better secured and someone from law enforcement should have been on the roof.

Fortunately, the shooter missed Trump’s head and only shot his ear.

Of course, some of the other bullets hit folks in the stands … killing firefighter Corey Comperatore and wounding 2 others.

With that in mind, former Secret Service Special Agent Michael De Geus says this was absolutely NOT a failure … if for no other reason than the fact Trump is still alive.