The shooter who tried to kill Donald Trump mounted a "sustained, detailed effort" to attack a major gathering of some sort before deciding to target the Republican presidential candidate at a Pennsylvania rally in July, FBI officials said on Wednesday.

Thomas Crooks, 20, searched more than 60 times for information about the Republican presidential candidate and his then-rival, U.S. President Joe Biden, before registering for the Trump rally in Butler, Pa., in early July, the FBI said.

"We saw ... a sustained, detailed effort to plan an attack on some events, meaning he looked at any number of events or targets," Kevin Rojek, the FBI's top official in western Pennsylvania, said in a telephone briefing to reporters.

Rojek said Crooks became "hyper focused" on the Trump rally when it was announced "and looked at it as a target of opportunity."

Rojek said the FBI has not yet been able to determine what motivated Crooks to try to assassinate Trump at the July 13 rally.

Crooks's computer activity showed he was interested in a mix of ideologies but did not show definitively that he was motivated by a particular left-leaning or right-leaning point of view, Rojek said.

FBI officials said they had not found any evidence that indicated that Crooks had worked with other people or had been directed by a foreign power.

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

The assassination attempt prompted questions about how Crooks was able to climb a nearby building and fire eight shots at the former president before being killed by a Secret Service sharpshooter. U.S. media have reported that the shooter was spotted by several rally attendees, local law enforcement officers and Secret Service snipers minutes before he started shooting.

Several congressional and government probes are examining the event's security measures. Kimberly Cheatle, the head of the Secret Service charged with protecting Trump, resigned over the agency's handling of the event.

The FBI, meanwhile, is investigating Crooks himself. Officials said they had gained some understanding of his mindset, even if they still did not know what motivated him.

Crooks searched for Trump's campaign events as early as September 2023, FBI officials said, and began searching in April for campaign events for both candidates near where he lived in western Pennsylvania.

He also searched for the dates of both the Republican and Democratic conventions, they said.

In the days leading up to the July 13 rally, he searched for information about the site, including where Trump would speak and details of the company that owned a nearby building from which he would later fire the eight shots, one of which grazed Trump's ear.

Video evidence shows Crooks was only on the building's roof for about six minutes.

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