BUTLER, Pa. (AP) — Former President Donald Trump is calling for unity and resilience after an attempt on his life added fresh uncertainty to a tumultuous presidential campaign. His opponent, President Joe Biden, implored Americans to “cool it down” in the final stretch of the closely contested race and to “resolve our differences at the ballot box.” A full day after the attack, the gunman’s motive was still a mystery, and investigators said they believe he acted alone. Biden ordered an independent security review of the shooting, which left one bystander dead and two others critically wounded. The FBI was investigating the shooting as a potential act of domestic terrorism. Trump arrived Sunday evening in Milwaukee for the Republican National Convention, which begins Monday.