The 20-year-old man who tried to shoot former US president Donald Trump searched online about the John F Kennedy assassination on the day he registered for Trump’s rally in Butler, a senior FBI official says.

“Analysis of a laptop that the investigation ties to the shooter reveals that on July 6, he did a Google search for ‘how far away was Oswald from Kennedy?’,” Pennsylvania FBI director Christopher Wray has testified to the US House Judiciary Committee.

“That is the same day that it appears that he registered for the Butler rally.

Wray told the committee on Wednesday that suspected shooter Thomas Crooks had become “very focused on president Trump and his rally” at the time.

Former president Kennedy was assassinated in Dallas, Texas, on November 22, 1963, by Lee Harvey Oswald.

Wray said Crooks, a nursing home aide, fired at least eight rounds from his rooftop position near the July 13 rally. The Republican presidential candidate suffered an ear wound, one rally attendee died and two others were wounded.

Trump has recently been seen sporting a bandage over the ear that he claimed was injured. But Wray cast doubt on whether any bullet actually hit the presidential candidate.

“There’s some question about whether or not it’s a bullet or shrapnel that hit his ear,” he told the committee.

Crooks used an AR-15 rifle with a collapsible stock, “which could explain why it might have been less easy for people to observe”, Wray said.

The motive for the shooting remains unclear.

Wray said it was frustrating that the investigation had not “yielded significant clues”. But he said investigators had seen “indications” that Crooks was interested in public figures and that he became “very focused” on Trump and the rally about a week before it was due to be held in Butler, Pennsylvania.

“The attempted assassination of the former president was an attack on our democracy and our democratic process and we will not and do not tolerate political violence of any kind, especially a despicable account of this magnitude,” Wray said.

“We’re going to leave no stone unturned. The shooter may be deceased but the FBI’s investigation is very much ongoing.”

Wray said many people had described Crooks as a loner and the list of contacts in his phone was short.

He told lawmakers that Crooks went to the campaign site on the morning of Trump’s rally. He stayed for about an hour, and then left to buy 50 rounds of ammunition.

Crooks later flew a drone about 180 metres from the stage where Trump was to speak, and live-streamed footage for about 11 minutes.

Wray said crude explosive devices recovered from Crooks’ car and home were designed to be detonated remotely.

Crooks had a transmitter with him at the time of the shooting, Wray said. But he said the FBI believed Crook would have failed had he tried to detonate the devices.

The hearing also focused on the increasingly tense political atmosphere surrounding the US presidential campaign.

“I have been saying for some time now that we are living in an elevated-threat environment. And tragically, the … assassination attempt is another example, particularly heinous,” Wray said.

Kimberly Cheatle resigned as director of the US Secret Service on Tuesday after bipartisan demands to quit over the failure to prevent the attempted assassination.

Judiciary Committee chairman Jim Jordan said he expected Wray to answer questions about what happened before, during and after the Trump shooting but expressed doubt about the FBI director’s answers even before questioning began.

“I’m sure you understand that a significant portion of the country has a healthy skepticism regarding the FBI’s ability to conduct a fair, honest, open and transparent investigation,” Jordan said.

Representative Jerrold Nadler, the panel’s top Democrat, condemned the shooting “unequivocally and unabashedly” but pointed to years of political threats and violence, and violent rhetoric from Republicans including Trump himself.

“If you think that this one assassin’s bullet was a bolt out of the blue, and not part of a wave of violence that has threatened this nation for years, then you have missed the point,” the New York Democrat said.

-with AAP