On the day of the shooting, the 20-year-old suspected gunman, Thomas Matthew Crooks, told his father that he wanted to go to the gun range, so his dad let him use his AR-15-style rifle, according to a source familiar with the matter.

Crooks would occasionally go target shooting and use his father's gun, the source told ABC News. Since this wasn't an unusual occurrence, the father didn't think anything of it at the time, the source said.

Crooks recently purchased ammunition both online and in person, the source said.

-ABC News’ Mike Levine

Secret Service Director Kim Cheatle will address the GOP-led House Committee on Oversight and Accountability at 10 a.m. on Monday, July 22.

"The United States Secret Service has a no-fail mission, yet it failed on Saturday," committee Chairman James Comer, R-Ky., said in a statement Monday.

"Americans demand answers from Director Kimberly Cheatle about these security lapses and how we can prevent this from happening again," he said.

The building where gunman Thomas Crooks scrambled onto the roof and opened fire was the staging area for the local police tactical team doing overwatch of the crowd, two law enforcement officials told ABC News.

Sources previously told ABC News that the building had been swept, but investigators are trying to determine whether roof access had been properly locked down.

The gunman gained roof access without a ladder, according to a source briefed on the preliminary findings.

-ABC News' Josh Margolin and Aaron Katersky

New video licensed by ABC News shows police investigating near the building where the gunman was perched on the roof a full minute before shots rang out.

In one video, at approximately 6:10 p.m. and 26 seconds, police look onto the roof as a man in white shorts points toward the building.

The next clip begins just before 6:11 p.m. Bystanders are seen fleeing the area and the first gunshots are heard about 30 seconds into the video.

-ABC News’ Victor Ordoñez and Christopher Looft