How do these police snipers not see the guy on the closest roof to Trump’s rally?

The trained police snipers on the roof of the building to the right did not see a strange man with camouflage gear and a military assault rifle climb onto the roof of the building on the left. And they also ignored eye witness warnings of him being there, such warnings given three to four minutes prior to Trump being shot at.

This is seconds before Trump was shot – note how the snipers are aimed at the building that the shooter was on. There is You Tube coverage of these snipers firing back immediately after Trump was shot.

So what we know briefly is: 1) an eyewitness saw the shooter on the roof and told police a few minutes before Trump was shot; 2) the shooter managed to climb onto the roof of the nearest building to Trump, unsighted, in camouflage gear and with a military-style assault rifle: 3) the police and secret service did nothing/saw nothing prior to Trump being shot; 4) immediately after he had been shot, the very same snipers saw the suspect and returned fire onto the suspect’s rooftop.