Former President Donald Trump was abruptly rushed off stage during a rally after shots were fired.
Trump’s campaign team has confirmed that he is in good condition.
A witness told the BBC they saw a man with a rifle crawling on a nearby roof before the shots rang out.
The attacker, a male, was shot and killed by a member of the Secret Service.
Video footage shows Trump, the leading Republican presidential contender, drop to the ground and then stand up with blood on the side of his face.
A shooter is dead, Butler County District Attorney Richard A. Goldinger told ABC News.
One bystander is confirmed dead, and possibly a second bystander is dead as well, Goldinger added.
US President Joe Biden condemned the shooting at the Trump rally and called on other Americans to do the same.
Further details on the incident are expected as investigations continue.