Most of the security personnel working at the Donald Trump rally in Butler, Pennsylvania, during which the Republican presidential candidate was shot, weren't even Secret Service, US Senator Josh Hawley has claimed, citing whistleblowers.

Shots fired from the roof of a nearby factory nicked Trump's ear, killed one audience member, and injured two more before the sniper was taken out by law enforcement.

"Whistleblowers tell me that MOST of Trump's security detail working the event last Saturday were not even Secret Service," Hawley, a Republican from Missouri, said on Friday, accusing the Department of Homeland Security of assigning "unprepared and inexperienced personnel" to the campaign rally.