A 20-year-old Pennsylvania man is the suspected shooter behind an "assassination attempt," on Donald Trump, the FBI has confirmed.


The shooter was identified as Thomas Matthew Crooks.


The US Secret Service confirmed Crooks, who fired multiple shots at the campaign rally in Butler, Pennsylvania, was killed at the event.

Special agent in charge of the FBI's Pittsburgh field office Kevin Rojek said authorities had not yet identified a motive.


"This evening, we had what we're calling an assassination attempt against our former president Donald Trump. It's still an active crime scene," Rojek said at a press conference.


A rally attendee died and two were critically injured in the incident, the FBI said.


Trump was seen with blood on his right ear surrounded by security agents on stage, moments after shots were heard at the rally on Sunday (AEST). The agents then rushed him off the stage as he pumped his fist in the air to a crowd.


Trump later said he was shot with a bullet that pierced the upper part of his right ear.


"I knew immediately that something was wrong in that I heard a whizzing sound, shots, and immediately felt the bullet ripping through the skin," he said in a post on social media.