U.S. Secret Service counter-sniper teams were unable to immediately see the shooter at a rally of former President Donald Trump because of an obstructed view caused by the tilted roof, The Washington Post (WP) reports. 

Based on a 3D model of the rally venue, the publication concluded that the Secret Service initially failed to see the attacker, Thomas Matthew Crooks, because of the slope of the roof of the building, on which he took up a shooting position. In addition, one of the counter-sniper teams was blocked by trees.

However, according to the newspaper, intelligence officers had already had their weapons drawn two minutes before Crooks opened fire. As former U.S. police officer Jason Lawless explained, this means they were notified of the threat.
A law enforcement official, on condition of anonymity, told WP that the sniper who eliminated Crooks held him at gunpoint for several moments, trying to see if he was armed and posed a threat, and fired seconds after the intruder began shooting.