All hell broke loose at Donald Trump’s rally in Butler, Pennsylvania, on Saturday evening when a gunman attempted to assassinate the former US President.

The assassination bid was foiled by alert Secret Service agents, who shot and killed the gunman. However, the attack left one bystander dead and another critically injured.

The would-be assassin’s bullet grazed Trump’s ear and left him bleeding. Photos and videos went viral of him defiantly pumping his fist moments after he was shot at.

Donald Trump, who is leading the Republican presidential race, was addressing his followers at a rally in Butler on Saturday (local time) when gunfire rang out. He grimaced and ducked before he was rushed off-stage by US Secret Service personnel. Blood was seen on Trump's face as he claimed a bullet pierced the upper part of his right ear.

The former US President was seen pumping his fist in the air, with the crowd cheering, as he was tucked inside a waiting vehicle.

US agencies identified 20-year-old Thomas Matthew Crooks of Bethel Park, Pennsylvania as the suspect in the attempted assassination of Trump at a campaign rally on Saturday. The FBI said it was working to determine a motive for the attack.

State voter records show that Crooks was a registered Republican, according to several US media reports. The upcoming November 5 election would have been the first time Crooks had been old enough to vote in a presidential race. Crooks lived about an hour away from where the shooting took place in Butler.