The gunman who tried to assassinate Donald Trump spent months researching him, along with other targets like Joe Biden, before deciding to shoot the Republican presidential candidate, the FBI has said.

The bureau said Thomas Crooks, 20, was involved in a “sustained detailed effort to plan an attack” before eventually becoming “hyper focused” on Mr Trump’s rally in Pennsylvania.

But Kevin Rojek, the special agent in charge of the FBI’s Pittsburgh field office, has also admitted experts have not yet been able to determine what motivated him.

Crooks’ computer activity showed he was interested in a mix of ideologies, but did not show a definitive motivation from a particular left-leaning or right-leaning point of view.

Mr Trump was struck in the ear when Crooks, who was positioned on the roof of a nearby building, fired eight shots before being killed by a Secret Service counter sniper.

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Two improvised explosive devices were discovered in Thomas Crooks’ car. Pic: FBI

This image provided by the FBI shows Thomas Matthew Crooks' rifle broken down as was likely done for transport and the backpack recovered at the scene in Butler, Pa., July 13, 2024. Crooks searched online for events of both former President Donald Trump and President Joe Biden and saw the Pennsylvania campaign rally where he opened fire as a "target of opportunity," a senior FBI official said. (FBI via AP)
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The rifle used in the assassination attempt could be disassembled and hidden in this backpack, also recovered from the scene. Pic: FBI

Video evidence shows he was only on the roof for about six minutes, killing one person and wounding two others when he opened fire at the event on 13 July.

Crooks, who left several explosive devices in his car, had searched for information about bomb components as early as 2019.

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Trump gunman’s motive ‘unclear’

This image provided by the FBI shows Thomas Matthew Crooks' rifle as recovered at the scene in Butler, Pa., July 13, 2024. Crooks searched online for events of both former President Donald Trump and President Joe Biden and saw the Pennsylvania campaign rally where he opened fire as a "target of opportunity," a senior FBI official said. (FBI via AP)
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The FBI recovered the rifle used by Thomas Crooks to shoot and injure Donald Trump in July. Pic: FBI

Crooks searched for Mr Trump’s campaign events as early as September 2023 and continued looking for rallies for both candidates near where he lived in western Pennsylvania, and the dates of both the Republican and Democratic presidential conventions.

Mr Rojek described how Crooks eventually became “hyper focused” on the Trump rally in Butler, Pennsylvania, when it was announced in early July “and looked at it as a target of opportunity”.

In the days leading up to the event, he researched information about the site, including where Trump would speak.

His online search history revealed questions and phrases like: “Where will Trump speak from at Butler Farm Show?”, “Butler Farm Show podium” and “Butler Farm Show photos”.

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The layout of the rally in Pennsylvania.

Body of gunman in Trump assassination attempt
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The gunman was on the roof of a building for about six minutes before opening fire

The FBI says it has already conducted nearly 1,000 interviews.

The new details suggest Crooks was a man who investigators believe had a strong interest in explosives, major events and prominent political figures, but they have not been able to assign a single motive.

“We have a clear idea of mindset, but we are not ready to make any conclusive statements regarding motive at this time,” Mr Rojek said.

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Officials say they have not found any evidence Crooks worked with other people, or had been directed by a foreign power.

There were also no traces of illegal drugs or alcohol in his system.

Meanwhile in an interview with talk show host “Dr Phil” McGraw, Donald Trump claimed that Joe Biden and Kamala Harris made it difficult for him to have adequate Secret Service protection at his campaign rallies.

The former president said: “He (Joe Biden) always made it very tough for us to have the proper number of Secret Service people.”

He added: “I don’t know whose fault it is, but I will tell you, I’d have these massive rallies with 50-60,000 people, more, and our people were always fighting to get more security, more Secret Service, and he knew that we didn’t have enough”.

Preliminary investigations determined there were significant failures by the Secret Service.