Butler: A shooting at Donald Trump's rally in Butler, Pennsylvania is being investigated as an attempted assassination of the former president and presumptive Republican nominee, law enforcement officials say.

Trump called Sunday for unity and resilience as shocked leaders across the political divide reacted to the shooting.

The Secret Service said it killed the suspected shooter, who attacked from an elevated position outside the rally venue.

Not long before shots rang out, rally goers noticed a man climbing to the top of a roof of a nearby building and warned local law enforcement, according to two law enforcement officials.

One officer climbed to the roof and encountered Crooks, who pointed his rifle at the officer. The officer retreated down the ladder and Crooks quickly took a shot toward former President Donald Trump, and that's when the US Secret Service counter snipers shot him, said the officials who spoke to The Associated Press on condition of anonymity to discuss an ongoing investigation. (AP)