In an effort to campaign, former President Donald Trump held a rally at Butler Farm Show Grounds, Pennsylvania on Saturday afternoon (July 13, US time).

The gunman fired multiple shots from an "elevated position" outside the rally, according to the Secret Service. Law enforcement sources told CNN the gunman was on the roof of a building just outside the rally site.

Mr Trump suffered only minor injuries in the assassination attempt. The gunman was killed by Secret Service agents and at least one bystander was killed.

At a news conference, Kevin Rojek, FBI special agent in charge of the Pittsburgh office, said it was “surprising” that the gunman was able to fire multiple shots at a Trump rally.

"We are still reviewing the security apparatus that the Secret Service deployed to determine what may have occurred," Mr. Rojek said.

“There will be a lengthy investigation into exactly what took place and how that individual was able to access the location, what kind of weapon he had. All of that is really days, weeks and months of investigation,” Rojek added.

The investigation will allow authorities to "look at what went wrong and how we can do better in the future," added Pennsylvania State Police Lt. Col. George Bivens.

The officer also explained the difficulties of security work: “To protect politicians, opening a place to the public and protecting that place against all possible threats from highly determined attackers is extremely difficult.”

Footage of the shooting shows Secret Service agents rushing to protect Mr Trump with their bodies and escort him into the motorcade after confirming the shooter had been neutralized.

Photos taken by CNN show that US Secret Service snipers also set up guards on rooftops near the rally.

Quang Anh (according to CNN)