AMNA NAWAZ: The assassination attempt on Donald Trump is raising major questions about the Secret Service and its security protocols.

Chief among them, how was a 20-year-old armed with an AR-15-style rifle able to obtain a clear line of sight to a former president?

President Biden has ordered an independent review.

This afternoon, Homeland Security Secretary Alejandro Mayorkas said that security had previously been enhanced for former President Trump and that independent candidate Robert F. Kennedy Jr. would also now receive Secret Service protection.

For more on this, I spoke earlier with Carol Leonnig, Pulitzer Prize-winning reporter for The Washington Post and the author of "Zero Fail: The Rise and Fall of the Secret Service."

Carol, welcome back to the "News Hour."

Thanks for joining us.

CAROL LEONNIG, The Washington Post: Glad to be here, Amna.

AMNA NAWAZ: So on this central question of how a gunman got up on a roof within a line of sight of a former president, fired several shots before the Secret Service could evacuate him, what does your reporting show about how that happened?

CAROL LEONNIG: You know, this is the central mystery and concern.

There are some answers and there are some things we're still trying to figure out.

But since 1963 and John F. Kennedy's assassination in Dallas from a gunman in a tall building, every Secret Service agent who's responsible for planning security for an event has lost sleep over the line of sight.

It's a -- it's Secret Service 101.

You plan for every possible piece of high ground that would give someone an ability to shoot at the president or other senior official that the Secret Service is protecting at a public event.

In this case, it's really obvious, Amna.

The Secret Service did not physically mitigate the line of sight here.

That means they didn't do what they often do at big, big public events.

They didn't roll in a 14-wheeler or a bus or a crane or a van or a banner and place it in between the high ground behind the stage, meaning behind the audience, and where Donald Trump was speaking.

That breaks up the line of sight.

They didn't use that physical blockade.

And what is unknown is what happened in terms of securing those buildings, and particularly a glass plant company that was right behind the - - and outside the perimeter of the crowd, where a shooter positioned himself and used a roof as a platform to shoot at Donald Trump.

What's unclear is, in that perimeter, where local police are usually assigned by the Secret Service to secure those areas, to check those buildings, to make sure no one is getting on top of them, what's unclear is what instructions the Secret Service gave those local police and whether or not they completed their mission.

It's a mystery right now how this critical breakdown could have happened.

AMNA NAWAZ: There's a remarkable piece of your reporting as well that tells the story about one of those local officers who climbed up on the roof, saw the gunman, and then went back down the ladder before the gunman began to fire a shot.

Tell us about that.

CAROL LEONNIG: Yes, this was a stunning find by one of my partners on this story, Isaac Stanley-Becker.

He called all around, as we did yesterday, to Butler local county and township police officers.

And one of them relayed to us that an officer who had been alerted by bystanders that there had -- was a suspicious man clambering onto this roof, that police officer went to look for the man.

He hoisted himself up physically with both hands up to the roof to see what was there.

And because he was using both his hands to hoist himself, he didn't have a weapon in his hand.

Unfortunately, the gunman did point his weapon, according to this officer, at the officer who was trying to check him out.

To save his own life, to protect himself, the officer dropped down and let go.

And within minutes -- or, actually within seconds, depending on the account, fired -- shots were fired from this rooftop by this gunman at the stage.

AMNA NAWAZ: Carol, you literally wrote the book about the Secret Service.

When we spoke about that book back in may of 2021, you told me agents had been telling you for a while about the culture of secrecy, about chronic underfunding as well.

You said that many of them were whispering to you they were worried about a president being killed on their watch.

Given all of that, was this inevitable in some way?

CAROL LEONNIG: You know, I have been really devastated, after a lot of reporting that I did in 2014 and 2015 about just episodic security failures and gaffes and major breaches.

After that reporting prompted an Oversight Committee investigation and then a blue-ribbon panel by the Obama administration making a series of recommendations to strengthen the Secret Service and ensure that its mission was not so broadly spread, that it really focused on the security of the presidents and the most important VIP officials who run the government, after all of that reporting, after all of those recommendations, many of those recommendations were never executed and implemented.

So, now where are we, Amna?

Ten years later, the Secret Service appears to be spread too thin once again, and in a situation where the basic rules of Secret Service 101 don't appear to have been followed.

AMNA NAWAZ: That is Carol Leonnig, national investigative reporter with The Washington Post and author of the book "Zero Fail."

Carol, thank you.

Appreciate your time.

CAROL LEONNIG: (AUDIO GAP) Good luck today.