"It is incredibly difficult to have a venue open to the public, and to secure that against any possible threat, against a very determined attacker," Pennsylvania State Police Lt. Col. George Bivens said of the Secret Service, "That's a huge lift to try and do it."  No Secret Service officials attended Saturday night's news conference. 

(CBS)

I vehemently disagree. This was abject security failure.

                                                                   (image NYT)

The site was not a concrete canyon, not even Dealey Plaza with lots of buildings.  Attendees at Trump rallies are forbidden to carry weapons of any kind. Therefore the area in front of the stage reasonably could be assumed to be friendly to Trump and secure. If a gunman with a handgun had been hiding in plain sight among other rally-goers, Secret Service could be forgiven. But a rifleman would have chosen the high ground of the nearest building immediately, and as a gift--IF the Secret Service left it unoccupied. And they did. There is video of Crooks, the shooter, before he fired, video taken by rally-goers who saw him. One woman can be heard yelling "What are you doing!" before he fired a single shot. There is video from rally-goers in front of the stage zooming in on the unusual attention of the SS snipers on the roof of the structure behind the stage. The SS snipers saw Crooks, had their rifle sights trained on him. And still managed to let him get off several shots. It is incomprehensible failure.