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

Security planners have reassessed and reexamined their plans for President Joe Biden's visit to Las Vegas this week in light of Saturday's shooting, a senior official briefed on the matter told ABC News.

Officials expect that there will be a heightened level of security and additional federal agents on site in Las Vegas, the official said.

Biden will be in Las Vegas from Monday to Wednesday for several events.

The House Oversight Committee will receive a briefing from the Secret Service on Tuesday, a source told ABC News.

Secret Service Director Kim Cheatle is then expected to appear for a public hearing in the House on July 22, the source added, citing conversations with the Secret Service. House Oversight Committee Chairman James Comer said on Fox News Monday morning that he's "100% confident" the hearing with Cheatle will happen.

The Secret Service will also brief GOP committee members later on Monday, a source familiar with House Committee on Homeland Security told ABC News.