WATCH: Video shows gunman shot on roof of nearby building in Trump rally

CNN report shows the gunman was a few hundred feet away from former President Donald Trump in Pennsylvania

Former President Donald Trump was about 400 to 500 feet (120 to 150 meters) from the suspected gunman during the shooting at his rally in Butler, Pennsylvania, CNN reported, citing geolocated positions.

Trump was injured in what the FBI described as an assassination attempt. The Secret Service confirmed that the gunman and at least one audience member were killed. The gunman was shot by Secret Service personnel, the agency said.

In a news conference, Kevin Rojek, FBI special agent in charge of the Pittsburgh office, expressed surprise that the gunman managed to fire multiple shots during the rally.

"We’re still working through the security apparatus that the Secret Service had in place, what potentially happened," he said. "There’s going to be a long investigation into exactly what took place and how the individual was able to get access to the location, what type of weapon he had. All that is really days, weeks, and months of investigation."

Lt. Col. George Bivens of the Pennsylvania State Police noted that the investigation would help identify any security failures and improvements for the future. "It is incredibly difficult to have a venue open to the public and secure that against any possible threat against a very determined attacker," Bivens added.

Law enforcement sources at the scene tell CNN a shooter was positioned on a building rooftop just outside the venue where former President Donald Trump was holding his rally in Butler, Pennsylvania.

Attendee Nico Mitchell captured video of the snipers behind Trump as he spoke. As shots rang out, the crowd was seen screaming and ducking for cover.

Mitchell, still shaken, told CNN, "I can’t even begin to describe what happened."