The gunman who tried to kill Donald Trump searched online for a member of the Royal family as he was planning his attack, FBI investigators have said.

Investigators told Congress that Thomas Crooks, 20, who was killed by US Secret Service after he fired at the former president at a rally in Butler County, Pennsylvania on Saturday, had searched online for various high-profile figures as he “scoped out” the target for assassination, including an unnamed member of the British Royal family.

He also searched for Christopher Wray, the director of the FBI, and Merrick Garland, the US attorney general.

Since Saturday, FBI agents have since been investigating a possible motive for the attack, including by downloading the content of two mobile phones he owned.

The devices contained images of Trump and Joe Biden, and the dates of both of the former president’s rallies and the Democratic National Convention, which will take place in Chicago next month.

The disclosure came as the US Secret Service faced new accusations of incompetence over its failure to protect Trump, who escaped with an injury to his right ear.

The agency was alerted to a “suspicious person” in the vicinity of the stage an hour before the attack took place, and Crooks was rejected from the venue after setting off a metal detector.

Law enforcement sources told ABC News that Secret Service agents then spotted Crooks on a rooftop 150 metres from the stage 20 minutes before he opened fire.

Several members of the public said they warned security staff at the event in the minutes before the attack, pointing out that he had climbed onto a nearby roof and appeared to be carrying a gun.

Kimberly Cheatle, the director of the Secret Service, is facing calls to resign after she admitted agents had not stationed a sniper on the roof because it was too slanted, and could be dangerous.

On Wednesday, she was accosted by senators at the Republican National Convention, who demanded an explanation for the security breach.

Video posted online by Marsha Blackburn, one of the senators, shows her and a colleague berating Ms Cheatle explaining the timeline of the attack and asking why Crooks was not stopped.

She replied: “I don’t think this is the forum to have this discussion.”

Alejandro Mayorkas, the Homeland Security Secretary, has admitted the Secret Service failed to prevent Crooks obtaining a “direct line of sight” on Trump from outside of the rally’s secure zone.

Crooks fired on Trump using an AR-15 semiautomatic rifle he had borrowed from his father, and had stored explosives in his car.

Mr Biden has ordered a full investigation into the failings, which is running concurrently with the FBI investigation into the shooting itself.

Investigators said Crooks appeared to be worried about his mental health, searching for “major depressive disorder” before carrying out the attack.

His former classmates have described him as a reserved student who was bullied by his peers.

Vincent Taormina, who knew Crooks at school in 2016, told Fox News he was “smug” about politics and had mocked him for supporting Trump.

“He just did not like politicians, especially with the choices that we had,” he said. “He did not like our politicians.”

“I brought up the fact that I’m Hispanic and, you know, I’m for Trump. And he said, ‘Well, you’re Hispanic, so shouldn’t you hate Trump?’

“He called me stupid – or insinuated that I was stupid.”