JUST IN: Rooftop location used by Thomas Matthew Crooks to take aim at Trump was police staging area for rally

Two law enforcement sources told ABC News that the building that Thomas Matthew Crooks occupied as he took his shots at Trump and others in the crowd was a staging area for a police tactical team conducting overwatch of the rally crowd. Sources familiar with the investigation into the incident told the outlet that the roof had allegedly been swept before the event, however, Crooks was still able to gain access to the area.

Crooks was able to gain access and was on the building for a short period of time leading up to the assassination attempt, enough time for a police officer to confront him and retreat when Crooks pointed his rifle at the officer. According to video reviewed by the outlet, police looked toward the roof for a full minute before the shots were fired.

After the police officer retreated, Crooks took several shots at Trump, striking him in the ear as well as hitting rally attendees behind him. Trump grabbed hold of his ear and dropped to the ground as Secret Service agents covered him.

Although Trump was not seriously injured, two people behind him in the crowd were critically injured and had to be hospitalized. In addition, 50-year-old Corey Comperatore was killed while he was shielding his family from the spray of bullets. Comperatore was a former voluntary fire chief.

Questions surrounding the security protocols at the event have led calls for Secret Service head Kimberly Cheatle to step down from figures such as Elon Musk.