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Building where gunman was found was officer staging area
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