The FBI director has cast doubt on whether former US President and Republican candidate Donald Trump was shot during an assassination attempt at a political rally in Pennsylvania.

Christopher Wray was briefing Congress on the attempted assassination of Trump in Butler on Wednesday when he made the "explosive statement."

"With former President Trump, there's some question about whether a bullet hit his ear, or whether it was shrapnel," Ray said.

"I don't know now whether that bullet, in addition to hitting Trump's ear, landed somewhere else," he added, meaning authorities did not find the bullet that was supposed to hit Trump's ear.

Ray did not doubt the incident and the assassination attempt, but he expressed complete ignorance and questions about the details of Trump being hit by a bullet aimed at his head.

Ray was addressing the security failures that allowed gunman Thomas Matthew Crooks to open fire.

Crooks, 20, killed firefighter Corey Comperatore, 50, and wounded two others, including Trump, during the shooting.

Since then, Trump has given accounts of the moment he was shot and was seen wearing a bandage over his ear.