Washington, United States – AFP

The gunman who attempted to assassinate Donald Trump at a campaign rally searched online for details about the November 1963 shooting of US 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.”

“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.

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.