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, said the gunman flew a drone over the venue where the former president was scheduled to speak about two hours before he took the stage in Butler, Pennsylvania, on July 13.

Wray told members of the House Judiciary Committee that investigators have not established a motive for the shooting but "we are digging hard because this is one of the central questions for us."

Trump survived the assassination bid, suffering a wound to his right ear, and a Secret Service sniper shot dead the suspected gunman — named as 20-year-old Thomas Matthew Crooks — less than 30 seconds after he had fired eight shots.

"With respect to former president Trump, there’s some question about whether or not it’s a bullet or shrapnel that, you know, that hit his ear," FBI chief Wray said.

Two rally attendees were seriously injured and a 50-year-old Pennsylvania firefighter was shot dead.

Wray said Crooks "appears to have done a lot of searches of public figures, in general" but that there was no clear pattern to the research.

"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," the FBI chief 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.

"That obviously is significant in terms of his state of mind."

The FBI director said no evidence has emerged so far that Crooks had any accomplices or co-conspirators and he seems to have been a "loner."

Crooks was perched on the roof of a nearby building and opened fire on Trump with an AR-style assault rifle shortly after 6:00 pm, as the Republican White House candidate was addressing the rally in Butler.

Drone flight

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 attack.

He said it was not flown directly over the stage but about 200 yards (metres) away.

The drone and its controller were recovered in the gunman’s car along with two "relatively crude" explosive devices, Wray said.

Another explosive device was found in Crooks’s residence.

Wray 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 rally-goers 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.

He purchased 50 rounds of ammunition on the day of the attack and visited a shooting range the previous day.