Minutes before the shooting, gunman Thomas Matthew Crooks was seen walking around near the magnetometers and people reported him as suspicious, according to law enforcement sources.

When someone in law enforcement started to approach Crooks, the 20-year-old backed up and faded into the crowd, according to sources.

Crooks then climbed on top of air conditioning units and pulled himself up to the roof of the building beside those units, sources said.

Once on the roof of that first building, he worked his way toward another taller, adjacent building that gave him the vantage point to fire in Donald Trump’s direction, sources said. He jumped and landed where he wanted to be and slowly made his way to the summit of the sloped building, sources said.

Secret Service agents heard radio traffic about a suspicious person and later heard another radio dispatch about some sort of confrontation with police, sources said.

When bystanders noticed Crooks, an officer climbed up, sources said. Crooks pointed his gun at the officer who fell to the ground, sources said.

There were police personnel and snipers from the local countersniper units in the building where Crooks eventually took up a position to fire at Trump, sources said. Investigators believe that could have led to confusion as Secret Service snipers were trying to determine whether there was a threat to Trump and where it might be coming from.

The Secret Service sharpshooters knew that building was being used by law enforcement and, investigators believe, that might have led to a split-second delay in reacting as the Secret Service snipers had to figure out whether the man on the roof was an officer or a suspect, sources said.

-ABC News’ Luke Barr, Jack Date, Pierre Thomas, Josh Margolin and Aaron Katersky

House Speaker Mike Johnson said Wednesday that the House will set up a bipartisan task force to investigate the shooting when the House returns from recess.

"I will set up on Monday a task force, special task force within the House. The reason we'll do it that way is because it's a more precision strike, it goes quicker," Johnson told Fox News. "There is not a lot of procedural hurdles and we’ll have subpoena authority with the task force, as well. It will be compiled of Republicans and Democrats to get down to the bottom of this quickly so the American people can get the answers they deserve."

The speaker also said he will call on Secret Service Director Kim Cheatle to resign.

-ABC News’ Lauren Peller

The Department of Homeland Security’s inspector general has opened an investigation into the Secret Service’s handling of the rally.

A new bulletin listed under "ongoing projects" on the DHS Office of Inspector General website said the project objective is to "evaluate the United States Secret Service's (Secret Service) process for securing former President Trump's July 13, 2024 campaign event."

Senators will receive an unclassified, all-senator briefing via phone call Wednesday afternoon, according to Senate Majority Leader Chuck Schumer’s office.

The Secret Service, FBI and Department of Justice will address the senators.

The House will also have an unclassified, all-member virtual briefing Wednesday afternoon, multiple sources said.