Washington DC - The gunman who attempted to assassinate Donald Trump at a campaign rally searched online for details about the November 1963 shooting of President John F. Kennedy in the days before the attack, the FBI director said Wednesday.

FBI chief Christopher Wray, testifying before a congressional committee, also said the gunman flew a drone over the area where the former president was scheduled to speak about two hours before he took the stage.

Wray told members of the House Judiciary Committee that investigators "do not yet have a clear picture of his motive" for the July 13 shooting, but "we are digging hard because this is one of the central questions for us."

He said the gunman, Thomas Matthew Crooks (20), "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."

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"A lot of the usual repositories of information have not yielded anything notable in terms of motive or ideology," he said.

"Starting somewhere around July 6 or so, he became very focused on former president Trump and this rally," Wray said, and he registered that same day to attend the campaign event in Butler, Pennsylvania.

"On July 6, he did a Google search for, quote, 'How far away was Oswald from Kennedy?'" he said, a reference to Kennedy's assassination by Lee Harvey Oswald.

"So that's a search that obviously is significant in terms of his state of mind."

The FBI director said there is no evidence "so far" that Crooks had any accomplices or co-conspirators.

Crooks opened fire on Trump with an AR-style assault rifle shortly after 6:00 PM as the Republican White House candidate was addressing the campaign rally.

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.

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US Secret Service Director Kimberly Cheatle resigned on Tuesday, a day after acknowledging 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 – sometime between 3:50 PM and 4:00 PM – on the day of the shooting.

He said it was not flown directly over the stage but some 200 yards away.

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 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.

He said the gunman purchased a ladder on the day of the shooting but appears not to have used it. Instead, he climbed onto the roof using some mechanical equipment on the ground and vertical piping.

Wray also said Crooks's AR-style gun had a collapsible stock, which may explain why he was not seen by rallygoers or members of law enforcement with the weapon before the shooting.

He said Crooks visited the rally site on at least three occasions, about a week before the shooting, for about 70 minutes on the morning of the rally and again that afternoon.