Questions are being raised over the security lapses at Donald Trump's infamous Pennsylvania rally.

Republican lawmakers have confirmed they plan to investigate how the shooter managed to get up on the roof of a nearby building and fire multiple shots into the crowd before being killed.

Senior Fellow at Defence Priorities in Washington DC Gil Barndollar says there are plenty of questions about the execution of security.

"It does seem strange, if not inexcusable, that there was a shooter on the rooftop of a building just 400 feet away from the stage - that seems crazy, on the face of it."

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