This clip -- which is making the rounds on social media -- is absolutely stunning for all the wrong reasons … because it shows the shooter moments before he set up his weapon, took aim and attempted to kill the ex-Prez, and A LOT of people saw it.

Check it out … the cameraman captured the whole scene and you can see 20-year-old Thomas Matthew Crooks apparently trying to lay low after he’d made his way onto the top of a building outside the main grounds of the rally -- but he didn’t do a very good job, because many noticed him and some even verbally tried flagging his location to cops on the ground who were walking around.

You eventually see Crooks army crawl his way along the roof, where he's able to line up his shot … firing a handful of rounds into the crowd and at Trump, but ultimately missing his target.

As we told you -- and as could be seen in video TMZ obtained -- Crooks was met with immediate crossfire from Secret Service agents who were seemingly monitoring all this from a different rooftop across the way. Crooks was killed, and an investigation is underway to determine more about his background and motive.

Still, this new clip is deeply disturbing -- and just like the caption suggests … there was quite a bit of time that passed where Crooks was somehow able to set up without any law enforcement intercepting before he pulled the trigger, something that is now widely being labeled an abysmal security failure.

When you see this, you can clearly tell people saw this guy getting into position and despite them trying to flag it to cops, it looks like the response time was slow/almost nonexistent.

It wasn’t until AFTER he fired that Secret Service kicked into high gear — and the question now is … did they have eyes on him prior to him shooting???