A prosecutor in the US county where Donald Trump was injured in an assassination attempt said Sunday it was "surprising" that a shooter was able to position himself on a nearby rooftop before opening fire.

"That's the most surprising thing to me, when we have a former president here, that a guy was able to get, as you say, 150 yards away, able to get up on a roof and be able to take a shot," said Butler County district attorney Richard Goldinger.

He told MSNBC that "I do know we had some law enforcement in that building, which is even more surprising that he was able to get up there," adding the unit was normally used by a glass manufacturing company.

The FBI is leading the investigation into the shooting in Pennsylvania on Saturday, which killed one bystander and critically injured two others.

A video published by US outlet TMZ appeared to show the alleged gunman lying on his belly on a sloping rooftop and aiming a rifle, before a quick string of shots is heard.

Butler County police said they had responded to "a number of reports of suspicious activity," after multiple witnesses said they saw the gunman before the shooting and notified authorities.

Goldinger said that law enforcement officials reacted "as soon as they could."

"I mean I don't think they're going to just take a shot at him, if you know what I mean, because they see him up on a building," he said.

The Secret Service has defended its security for Trump -- the Republican candidate for November's election -- and said it had recently added resources for his protection.

The FBI said in a news conference early Sunday that it was "surprising" the gunman had been able to get off so many shots because he was "neutralized" by counter-snipers.