AMNA NAWAZ: The Secret Service is under intense scrutiny after the attempted assassination on former President Trump.

A homeland Security and congressional probe are under way, and the agency's director, Kimberly Cheatle, is expected to testify before the House Oversight Committee next week.

Yesterday, here on the floor of the Republican National Convention, lawmakers confronted Cheatle as she walked through the venue.

MAN: Stonewalling!

SEN. MARSHA BLACKBURN (R-TN): This was an assassination attempt.

You owe the people answers.

AMNA NAWAZ: For a closer look, I'm joined now by Anthony Cangelosi, a former Secret Service agent and a lecturer at the John Jay College of Criminal Justice.

Mr. Cangelosi, welcome.

Thanks for joining us.

ANTHONY CANGELOSI, Former U.S. Secret Service Agent: Thanks for having me.

AMNA NAWAZ: So the more we have learned about what happened here, the more questions it seems to raise.

Knowing as we do now that the shooter was able to get less than 500 feet within a former president to climb on top of the building, get a line of sight to him, and get off those shots, do you agree when lawmakers look at this and say this was a failure of the Secret Service?

ANTHONY CANGELOSI: Yes, it's undoubtedly a failure.

When I first started watching video footage of it, my first reaction is, where were these shots coming from?

How can this be?

And as the information started to come in, realizing this was only 150 yards with a clear line of sight to the podium where the president was going to be speaking, and that they didn't have an agent or some kind of armed law enforcement officer on the top of that roof, that's a critical error on the part of the planning.

And that gap in security could have resulted in someone climbing that roof and shooting indiscriminately into the attendees on that venue.

And there could have been a lot more death than there really was that day.

AMNA NAWAZ: Well, there have been some reports I'm sure you have seen that local police with whom they were coordinating had told the Secret Service they didn't have enough people to station a patrol car outside that building that we know the gunman was able to get access to.

Could that one factor have changed everything here?

ANTHONY CANGELOSI: Yes, there's a lot of planning that goes into these events.

They go into the -- they start planning these events days in advance.

And if that was known, and it should have been known, there had to be some kind of way of getting -- whether it was the local police or the Secret Service getting an agent to stand the post on that roof to deter any bad actors like the shooter from trying to get up there and trying to either kill President Trump or kill people in the venue.

This is something that proper planning was easy -- it was easy to avoid.

AMNA NAWAZ: There's questions around the planning, and there's also questions about communications in the moment.

As the details have come out through reporting, it appears to be that local police had actually identified the shooter as someone who looked suspicious.

They did not know he was armed, that they notified Secret Service.

We understand police were pulled to try to find him and they lost track of him.

And in that time, just eight minutes after President Trump took the stage, the gunman began to fire those shots.

If that timeline is proven to be correct, what should have been happening in there that didn't?

ANTHONY CANGELOSI: Yes, there are a lot of questions, like you said, if that timeline proves to be correct was communication crisp to the Secret Service command post?

What actions were actually taken?

Secret Service can sometimes send a P.I.

team, a protective intelligence team, out there to interview the suspect or a countersurveillance unit team to interview the suspect.

Again, I don't know where everyone was positioned on that day.

The other thing is, a police officer who's out there could approach him as well.

My question is, well, why didn't the police officer that saw him approach him and question him if he really thought he was suspicious?

AMNA NAWAZ: On that point, and to clarify here, when there is this kind of coordination between Secret Service and local police, as we understand does happen, where does the accountability end up?

Is it Secret Service's responsibility to make sure local police are doing what they're supposed to?

ANTHONY CANGELOSI: Look, as part of the planning stages, the Secret Service is coordinating with the local police department.

And communication is always a critical aspect of that preplanning stage.

You have got to try to get everybody on the same sheet, so as what their duties and responsibilities are.

Ultimately, it lies with the Secret Service.

The protection of the president, former president, the venue itself, they're ultimately responsible.

But it is true that, because of manpower issues, the local police department generally secures the outer perimeter of the venue.

AMNA NAWAZ: So, as we mentioned, the Secret Service director is expected to testify on Capitol Hill next week.

What questions do you think she needs to answer in that testimony?

ANTHONY CANGELOSI: Yes, I have been thinking about that.

One of the questions I would ask is, who's the highest official in the United States Secret Service that approved that site, site survey plan?

And why did they approve the plan without someone positioned on the top of that roof?

It seems evident to everybody.

I don't think you're going to find another retired Secret Service agent who doesn't think that was a critical flaw in the plan.

AMNA NAWAZ: That is Anthony Cangelosi, former Secret Service agent, lecturer at the John Jay College of Criminal Justice.

Mr. Cangelosi, thank you.

Good to speak with you.

ANTHONY CANGELOSI: Thank you.

You have a good evening.