Two sources told NBC News that the Secret Service flagged a building near the rally site as a security concern. Investigators will be scrutinizing how a gunman was allowed to scale it.

The rooftop where a gunman shot at former President Donald Trump during a campaign rally was identified by the Secret Service as a potential vulnerability in the days before the event, two sources familiar with the agency’s operations told NBC News.

The building, owned by a glass research company, is adjacent to the Butler Farm Show, an outdoor venue in Butler, Pennsylvania. The Secret Service was aware of the risks associated with it, the sources said.

“Someone should have been on the roof or securing the building so no one could get on the roof,” said one of the sources, a former senior Secret Service agent who was familiar with the planning.