Trump assassination attempts probe leaders concerned over Secret Service "culture of silence" / Lessons from a forgotten presidential assassination attempt / From Lincoln to Trump, America's dark history of presidential assassination attempts / World leaders condemn assassination attempt on Trump: 'Tragedy for democracy' / A look at past presidential assassination attempts in U.S. history / From Lincoln to Trump: A timeline of assassination attempts on US leaders / Assassination attempts on US presidents throughout history / How attempted Trump assassination could alter political rhetoric / How Secret Service protocol has changed amid presidential assassination attempts through the years / A look at the history of presidential assassination attempts in America