After an assassination attempt on Donald Trump on Saturday, former U.S. Secret Service and Federal Bureau of Investigation (FBI) agents are hoping the investigation uncovers what really happened.

Former U.S. Secret Service agent Mark Lowery says he believes the Secret Service team did a "wonderful job" covering and removing Trump from the stage where he was speaking, but he questions why the shooter's location on the rooftop of a building wasn't secured.

As part of the planning process, the Secret Service would've visited the entire site at least several days in advance of the event, he noted.

Reports from the scene suggest the shooter was about 122 metres from Trump as he spoke at the Pennsylvania rally. Lowery, who is no longer with the service but previously worked as a special agent in charge, told CTV News Channel that the Secret Service would have led the overall planning for securing the area at the rally.

As the FBI investigates the attempted assassination of Trump as a potential act of domestic terrorism, authorities identified Thomas Matthew Crooks of Bethel Park, Pa., as the shooter, The Associated Press reported.

Crooks, 20, was shot dead by the Secret Service. The FBI said it believes Crooks acted alone. He was registered as a Republican but federal campaign finance reports also show he gave US$15 to a progressive political action committee on Jan. 20, 2021, the day Biden was sworn into office.

Lowery said a counter sniper team would've taken into consideration the building where the suspected shooter was found, so a major question that the team will have to answer during the investigation is how someone with a weapon was able to get to it.

"That building is critical threat level even though it's outside the perimeter," he said. "What happened? Why wasn't it covered? Did someone not execute their job according to the plans? I too am waiting for the investigation to determine what actually happened."

Michael Harrigan, former FBI agent and threat assessment expert, echoed Lowery's comments, as the service was ultimately responsible for the presidential candidate's safety. But whatever the issue was must be disclosed to the general public, he said.

"The study that's done is really going to tell us a lot about what went wrong and we all need to know it – the country needs to know it, because the country needs to have confidence in the Secret Service," Harrigan told CTV News Channel. "The Secret Service is a highly proficient organization. I mean, look at the rarity of these events, and they do a phenomenal job stopping threats in advance of when they materialize."

Harrigan said it will take weeks, months or even longer to truly understand what the shooter's motivation was, but noted law enforcement has been "grinding hard" to find information and intelligence.

Watch the full interview above for more on what Lowery noticed as he watched coverage of Saturday's events.

With files from The Associated Press