There are fast moving developments after the assassination attempt tonight.

Remarkable new images here.

The alleged gunman seemed climbing to the roof just minutes before opening fire at that rally.

You can actually hear people in the crowd yelling to an officer pointing him out.

And now we learn that law enforcement was actually using that building as a staging area and how the gunman climbed onto the roof the wall outside that building.

ABC's Pierre Thomas tonight with the exclusive interview with the head of the Secret Service and what she's saying now.

Stunning new video tonight showing the moment Donald Trump's would-be assassin climbed onto the roof of that building just two minutes before he opened fire.

You can hear bystanders desperately trying to alert police.

Authorities now using that video as part of the investigation.

And tonight ABC News has learned that heavily armed local police were stationed inside that building and Miss Thomas Crooks climbing up the outside of the building to the roof.

The same building was being used as a staging area for local police according to two law enforcement officials.

And Butler Township officials are confirming ABC News' reporting that a local police officer climbed up the roof and confronted Brooks moments before the shooting.

The officer grabbed hold of the edge of the building, pulled himself up to be able to look.

And according to what he's told me, shooter turned, rifle was pointed at the officer, officer defensive move, kind of ducked, fell, lost his grip on the building.

Investigators tonight trying to determine whether roof access had been properly locked down.

The shooter climbing seemingly unimpeded about 400 feet from the stage with a direct line of sight on the former president.

Take a look at what happened.

Washington lawmakers demanding answers, calling on the Secret Service director to appear before Congress.

We sat down with Director Kim Cheadle today for an exclusive interview.

What was your reaction when you saw the events unfold on Saturday?

Shock and then concern, obviously, for the former president.

This is an event that should have never happened.

Who is most responsible for this happening?

What I would say is the Secret Service is responsible for the protection of the former president.

So the buck stops with you.

The buck stops with me.

I am the director of the Secret Service.

It was unacceptable and it's something that shouldn't happen again.

The president and Homeland Security secretary said today they had 100 percent confidence in you.

But there are some members of Congress calling on you to resign.

I appreciate the secretary's comments and we're going to continue to be transparent and communicate with people.

You plan to stay on?

Absolutely.

I do plan to stay on.

Director Cheadle says she will not wait on an independent review of the security failure.

She says she plans to make immediate changes.

We learned today she's already expanded the tactical or SWAT support for former President Trump.

David.

Pierre Thomas here in Milwaukee with us.

Pierre, thank you.