An apparent assassination attempt has many questioning how a shooting that wounded former U.S. president Donald Trump was even possible.

At a campaign rally in Butler, Pa., on Saturday, a gunman on a nearby roof fired multiple shots at the stage where Trump was standing. The presumptive Republican presidential nominee said he was nicked in the right ear.

Ken Gray, a former agent with the U.S. Federal Bureau of Investigation (FBI) and a senior lecturer at the University of New Haven in Connecticut, shared his thoughts with CTV News Channel on Saturday’s shooting, weighing on the agents’ performance and what this might mean for future rallies.

U.S. Secret Service reaction time

From Gray’s perspective, the service performed a “good” and “competent” job by throwing their bodies on Trump to protect him from any more shots fired, he said

“Nonetheless, the counter sniper team may have had eyes on this shooter and did not react to that. So that’s something that will have to be looked at as this investigation goes on,” Gray said.

The agents continued to swarm Trump on the ground near the podium until it was confirmed the suspected shooter – identified by the FBI as 20-year-old Thomas Matthew Crooks – was killed.

Witness and law enforcement accounts suggest Crooks was killed seconds after the shooting, and Gray speculated on what was being done during that time, based on his experience.

“When they are looking at this person that is crawling across the top of the building, they have to first identify him and identify him as a threat, and not potentially other law enforcement,” Gray said. “But once they have identified him, they may have been trying to direct ground agents or ground officers to the area instead of shooting him straight out.”

Gray posited the service could have acted faster, possibly taking Crooks down before any shots were fired, but he said that is only something an investigation can determine.

What's next for Trump?

With the Republican National Convention still proceeding as planned in Milwaukee, Wis., on Monday, Gray says there will likely be an increased security presence.

As for remaining campaign rallies until the Nov. 5 election, Gray says it is possible Trump will hold fewer events outdoors.

“Open-air venues are harder to protect because of the possibility of a shooter being outside the protected area,” Gray said.

The former FBI agent added the Secret Service will review Saturday’s rally to determine what went right and how they can correct what went wrong for future events.

How did the shooter get on the roof?

While the ongoing investigation has yet to determine how exactly the shooter made it to the roof, Gray mentioned a site survey would have been conducted prior to Trump’s rally.

“One would think that in doing the site survey before this event, that would have been identified as a potential problem area and people should have been posted on there, in my opinion,” Gray said. “So it’s a question of manpower, I think. This (was) only 450 feet (137 metres) away, 150 yards. That was a prime location for a shooter to be.”

This is the same distance U.S. Army recruits must hit to qualify with the M16 assault rifle in basic training, and the gun the shooter had – an AR-15 – is the semi-automatic civilian version. The range prompted speculation that the shooter might have military experience, but no records have been found.

So far, investigators say they have not determined a motive behind the shooting.

With files from The Associated Press and Reuters