Washington DC - The US Secret Service plans to increase its use of surveillance drones following the attempted assassination of former president Donald Trump, the agency's acting director said Friday.

"We did not have a drone on site" at the July 13 campaign rally in Butler, Pennsylvania, where a gunman opened fire on the Republican White House candidate, said Ronald Rowe, who took over after the previous director resigned.

Trump was slightly wounded in the right ear, two rally attendees were seriously injured, and a 50-year-old Pennsylvania firefighter was killed when the gunman, Thomas Matthew Crooks, fired eight shots from a nearby rooftop.

Crooks (20) was shot dead by a Secret Service counter-sniper on a building behind the stage where Trump had begun speaking.

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"We should have had better coverage on that roofline" from where Crooks opened fire, Rowe told a press conference.

"We thought we might have had it covered with the human eye," he said. "But clearly, we are going to change our approach now, and we are going to leverage technology and put those unmanned aerial systems up."

Crooks flew a drone near the rally site for 11 minutes about two hours before the attack, according to the FBI.

Rowe, who took over as acting director on July 23, said the assassination attempt was a "dark day" for the country and the Secret Service takes "full responsibility" for the "mission failure."

He said multiple investigations were being conducted into the assassination attempt, and Secret Service employees would be held accountable if there were any policy violations.

Rowe also said the Butler event was the first time Secret Service counter-snipers had been deployed to support the former president's security detail.

"Looking back, you know, it was very fortunate that we did."

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He said state and local counter-snipers had been present at previous Trump events.

Testifying earlier this week before a Senate committee, Rowe said Trump's security detail and the sniper teams were not aware there was a gunman on a roof until he opened fire on the former president.

He said the Secret Service had been made aware by local law enforcement that they were looking for a "suspicious individual," but "suspicion had not risen to the level of threat or imminent harm."