Trump rally shooting raises concerns of political violence. Here's a look at past attacks against U.S. presidents and candidates.

A shooting at a Pennsylvania rally on Saturday that injured former President Donald Trump has heightened concerns about political violence and increased awareness of past attacks and assassination attempts against presidents and candidates. 

In a social media post shared on Saturday night, Trump thanked law enforcement officials for their quick actions after he was "shot with a bullet that pierced the upper part of my right ear."

The Butler County district attorney confirms to CBS Pittsburgh station KDKA that two people are dead — the gunman and an audience member. Two law enforcement sources told CBS News the gunman was killed by a Secret Service sniper. Two other attendees are in critical condition.