WASHINGTON, United States (AFP) – The 20-year-old gunman who attempted to assassinate Donald Trump at a campaign rally flew a drone over the area about two hours before the former US president began to speak, the FBI director said Wednesday.

FBI chief Christopher Wray, testifying before a congressional committee, also said the investigation into the July 13 shooting had not yet revealed a motive for the gunman.

Thomas Matthew Crooks opened fire on Trump with an AR-style assault rifle shortly after 6:00 pm as the Republican White House candidate was addressing a campaign event in Butler, Pennsylvania.

Crooks, who was perched on the roof of a nearby building, was shot dead by a Secret Service sniper less than 30 seconds after firing his first shots.

Trump was wounded in the ear, two rally attendees were seriously injured and a 50-year-old Pennsylvania firefighter was shot dead.

US Secret Service Director Kimberly Cheatle resigned on Tuesday, a day after acknowledging that the agency had failed in its mission to prevent the assassination attempt.

Wray said Crooks flew a drone over the rally area for around 11 minutes — between around 3:50 pm and 4:00 pm — on the day of the shooting some 200 yards (meters) away from the stage where Trump was scheduled to speak.

The drone and its controller were recovered in the gunman’s car.

Two “relatively crude” explosive devices were also found in the car along with another at the gunman’s residence, Wray said.

The FBI director said the devices had the ability to be detonated remotely and the gunman had a transmitter with him when he was shot.

“It looks like, because of the on-off position on the receivers, that that if he had tried to detonate those devices from the roof, it would not have worked,” he said. “But that doesn’t mean the explosives weren’t dangerous.”

Asked how many shots Crooks fired, Wray said eight cartridges had been recovered on the roof.

Wray told members of the House Judiciary Committee that investigators “do not yet have a clear picture of his motive.”

“We’re not seeing that yet, but we are digging hard, because this is one of the central questions for us,” the FBI chief said.

He said the gunman “appears to have done a lot of searches of public figures, in general” but “there doesn’t seem to be a whole lot of rhyme or reason to it so far.”

“Sarting somewhere around July 6 or so, he became very focused on former president Trump and this rally,” Wray said.

“On July 6, he did a Google search for quote, ‘How far away was Oswald from Kennedy?'” he said, a reference to the 1963 assassination of president John F. Kennedy by Lee Harvey Oswald.

There is no evidence at this time that Crooks had any accomplices or co-conspirators, the FBI chief said.