Former Secret Service agent on what security breakdowns could have led to attempted assassination of Trump

Hours after a shooter attempted to assassinate President Donald Trump at a rally in Pennsylvania, multiple federal agencies started working together to develop a timeline of the shooter's movements in the hours and days leading up to Saturday's shooting.

The Secret Service has been put under a high level of review as authorities try and figure out how security was breached at the rally, putting a former president and the presumptive GOP presidential nominee's life at risk.

RELATED STORY | Secret Service under intense scrutiny after assassination attempt on Donald Trump

Former Secret Service Special Agent Melanie Burkholder, PhD, told Scripps News that when Secret Service agents surround Donald Trump on the rally stage as shots rang out — forming that human barrier — that it was the right thing to do.

Burkholder said Trump's instinctive reaction to duck down behind the lectern was also the correct move in that situation. In those intense first moments, it was uncertain from where bullets were coming from, so getting down and behind the human barrier as agents quickly led the former president into a vehicle in his motorcade was the best route to bring him to safety.

RELATED STORY | FBI believes Trump rally shooter used father's gun, acted alone

The FBI is now the lead agency charged with investigating and forming a timeline into the actions and motivations of the shooter, named as 20-year-old Thomas Matthew Crooks of Bethel Park, Pennsylvania. The agency believes Crooks acted alone, but they were not immediately clear on any motive for the shooting.

The FBI believes Crooks used his father's gun as he fired shots from the roof of a nearby building.

Burkholder said, "The question becomes how did this person penetrate a perimeter? Why wasn't that building secure? Why wasn't it swept and maintained secure throughout the event, knowing a rifle could potentially assassinate a former president?"

She added, "That's the biggest question. Were the resources requested? Were they denied? Did they go unregarded? What happened? And then how was it communicated to the counter snipers to train on him? How did that get communicated? That is a missing piece. I've heard wild reports about this, but I'm not going to spread wild reports until they're founded. So, I think we have basic questions that need to be answered, and someone needs to be held accountable."

RELATED STORY | Secret Service: No change to RNC security plan after Trump assassination attempt

Burkholder said she believes Secret Service would have been responsible for the parts of the perimeter that would have included the structure from where the shooter fired from its roof.

"The building should have been rendered secure, and nobody should have been going in or out of the building," she said.

However, she added that the Secret Service relies on local partners to do some of that work.

"There's only about 7,500 Secret Service agents, that's not enough to fulfill our mission. So, we rely heavily on local law enforcement, sheriff's departments, military to supplement our security details," she said.

"Our security plan, our operational security plan is based upon threat level, from what I understand ... this was Trump country so perhaps there was not an increased threat level there. I'm not sure, but I do know if you're 140 yards away from a president and you have a clear line of sight, you're asking for disaster, and that's exactly what happened," Burkholder said.