The FBI on Monday morning told reporters that would-be Trump assassin Thomas Crooks accessed the roof of a building near former President Trump's July 13 rally in Butler, Pennsylvania, by climbing up HVAC equipment and piping.

Crooks then traversed multiple rooftops before he found his shooting position on top of a building owned by American Glass Research (AGR), located near the Butler Farm Show fairgrounds about 150 yards from where the former president spoke at his rally.

The FBI hosted a press conference call with reporters more than two weeks after 20-year-old Crooks fired at Trump with an AR-15 during the rally in an assassination attempt that nicked the president's ear but left one man dead and two others hospitalized with gunshot wounds.

While investigators determined that Crooks did purchase a ladder just hours prior to the assassination attempt, he left it at his residence in Bethel Park and did not use the ladder at the rally later on.

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It is still unclear how Crooks evaded security even after being noticed by law enforcement more than an hour before shots rang out, but the FBI said more than 300 agents and staff are working "round the clock" to gather facts and put together a clearer timeline of Crooks' actions. 

Investigators located eight rounds on the roof where Crooks fired from, the agency said during the call.

FBI Director Christopher Wray testified during a July 17 congressional hearing that Crooks had been at the rally site for about 70 minutes the morning of the assassination attempt. He also flew a drone about 200 yards away from where the former president would be speaking, Wray said. 

Law enforcement first reported seeing a suspicious person near the rally site around 5:10 p.m. on July 13 — an hour and one minute before Crooks began shooting. Local law enforcement notified command about the suspicious person and received confirmation that the Secret Service was aware of his presence.

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Over the next hour, law enforcement identified a building of interest, and snipers took photos of Crooks and his bicycle left near the rally grounds. Republican Missouri Sen. Josh Hawley, who was briefed on the assassination attempt, previously told Fox News Digital that a sniper "had eyes on" the suspicious person about 20 minutes before Crooks began firing.

Trump took the podium around 6 p.m. — an hour after he was scheduled to speak. Eleven minutes later, Crooks fired multiple rounds, killing 50-year-old Corey Comperatore and seriously wounding 57-year-old David Dutch and 74-year-old James Copenhaver. Dutch was hospitalized and has since been released, while Copenhaver remains hospitalized.

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FBI officials are still working to determine Crooks' motive behind the assassination attempt that rocked Butler and neighboring townships. They are also still working to determine whether Crooks planned with any coconspirators, though the agency clarified on Monday that there are no signs to indicate there were others involved.

Trump is expected to return to Butler at some point for another rally, but he has not released any details on when that will be.

This is a developing story. Check back for updates.