It's the Secret Service's biggest crisis in decades.

How did a 20-year-old gunman climb a rooftop with a rifle and fire shots at former

President Donald Trump from less than 150 yards away?

This is a colossal failure for the Secret Service.

The Secret Service isn't just responsible for the safety and security of the protectee. They're also there for the safety and the security of the people that are showing up to these events.

So this is tragic all around.

Here's what we know about the security gaps that led to the assassination attempt on

Trump.

In a video taken by a bystander shortly before the shooting, the gunman, Thomas

Matthew Crooks, could be seen crawling on a roof as people were heard pointing him out.

Dangerous people.

And a witness told the BBC.

The police are down there running around on the ground.

We're like, hey, man, there's a guy on the roof with a rifle.

But those alerts, if heard by any officers, didn't seem to make it up the chain back to the command center.

That's a massive communication failure.

When I was overseeing sites, we would have Secret Service agents right next to police with police radios.

If there's something that's going on, boom, everybody knows instantly.

Quite frankly, if the detail had known there was an armed person climbing up on a roof, they would have evacuated the president right away.

No questions asked.

Beyond that, the Secret Service said it relied on local law enforcement to secure the building that Crooks climbed.

Typically, Secret Service and local police operate together in concentric circles.

Closest to the principal, in this case Trump, are teams of highly trained Secret

Service agents. In the middle perimeter, there's usually a mix of Secret Service and local law enforcement who might cover portions of the motorcade and any pre-event screening. And in the outer perimeter, security largely falls on local law enforcement. In Butler, the building the shooter climbed was technically outside of the rally venue, and the Secret Service didn't sweep it.

It fell through the cracks, which still falls to the Secret Service because it means nobody checked to make sure the plan was, in fact, implemented correctly.

According to the Butler County Sheriff, right before the shooting, a local officer climbed onto the building and saw Crooks, but dropped to the ground when Crooks aimed his rifle at him. In an interview with ABC News, the Secret Service director, Kimberly

Cheadle, said local officials were inside the building when Crooks scaled it, but said. The buck stops with me.

I am the director of the Secret Service.

There are also questions about why counter sniper teams located behind Trump didn't take out the shooter earlier, even though they appeared to be facing in his direction. Once the shooting started, they had rounds on target almost instantly.

So that tells me that that shooter had come to their attention and they were in the decision loop.

The investigations into what went wrong will also focus heavily on what happened before the event even took place.

The Butler rally was announced by the Trump campaign 10 days ahead of time, which experts say gave the Secret Service plenty of time to do site advance planning and potentially even eliminate the line of sight from the building where the gunman was perched to Trump's stage.

In this situation, you actually had an event site that was being built, so they're there to find out what the dimensions are going to be, what the stage is going to look like, where it's going to be located.

And then based on that information, they are then looking at potential threats, including areas of high ground and how to best mitigate those threats with the overall security plan.

Former Secret Service agents say the buildings near Trump's stage in Butler would be a glaring risk and not something that could have been missed by advance agents.

We lost a president in 1963.

How did it happen?

Man up on a roof with a rifle.

And guess what?

The protocols that went into place from 1963 until this day is that we prioritize high places because we understand a man with a rifle, woman with a rifle, whoever with a rifle, if they have any level of expertise, they can kill a president very quickly. One thing law enforcement experts say was done correctly, despite widespread criticism on social media, was the evacuation of the former president after he was shot. The reaction from the shift inside that inner perimeter was very quick.

And as it's happening, if you if you listen closely, you can hear the instructions to former President Trump to get down, and that is so that they can cover him effectively. That was done very well.

Let me get my shoes on.

What you can't hear is what's being said to the agents through the radios from the command post, maybe the counter sniper team, you know, because there's other things going on. So they're going to do what they're told.

And if they're told now it's safe to move, then that's what they're going to do.

While experts say it's not ideal from a security perspective that Trump stopped to pump his fist, the shift did a good job getting him to the car known as the spare.

But the team near the spare?

They were clearly confused.

You could see it. You can't hide it.

They didn't know what to do.

President Biden has been briefed about the assassination attempt and has pledged a quick probe led by the FBI.

I'm instructed that this investigation be thorough and swift, and the investigators will have every resource they need to get this done.

The House Oversight Committee is also investigating what went wrong and has called on

Cheadle to testify. In a statement, Cheadle said the Secret Service is working with all involved federal, state and local agencies to understand what happened, how it happened and how we can prevent an incident like this from ever taking place again.