Thomas Matthew Crooks likely scouted the Butler Farm rally site and planned his attack extensively before attacking a Trump rally in Butler County, Pennsylvania, Saturday in a failed attempt to kill the former president, according to a retired Secret Service agent and security expert.
"It’s clear now the gunman did an advance of the target before the event," said Bill Gage, who is now a consultant at Safehaven Security Group.
"[Arthur] Bremer, [Lee Harvey] Oswald and [John] Hinckley, they all scouted before," he added, referring to men who attacked Presidents Richard Nixon, John F. Kennedy and Ronald Reagan, respectively.
Crooks, 20, was carrying a range-finder and behaving suspiciously when he showed up "in or just-outside" the venue Saturday before former President Donald Trump took the stage, according to law enforcement sources. Other evidence that emerged Tuesday: He purchased ammo hours before the rally. Crooks also bought a ladder at Home Depot, Fox News confirmed.
"He knew that there were elevated positions there i.e., those buildings off in the distance and that he would need a ladder to access," Gage told Fox News Digital. "It is being reported that he didn’t use it – but the purchase indicates to me that there was some advance planning involved."
Crooks knew he'd need a rifle, not a handgun, he said. The killer packed explosive materials into his vehicle. He also brought body armor, which he didn't wear when he climbed the roof – a choice likely aimed at avoiding detection, Gage said.
Crooks didn't use the ladder. He climbed up an air conditioning unit to get onto the roof, ABC News reported earlier.
WATCH: Exclusive video shows new perspective of Trump attempted assassination
"But buying a ladder is very telling," he added. "Sometime between the announcement of the event and Trump taking the stage, he scouted out the location and decided that a rooftop attack with a rifle was the best plan."
David Katz, a former DEA special agent and federal firearms instructor who is now the CEO of Global Security Group, said the range-finder Crooks was carrying is also a sign of the preparations he took ahead of time.
"That tells me this kid not only practiced but was aware of how to sight-in his rifle at that distance and probable declination of angle," he told Fox News Digital. "It tells you the exact distance to your target, and the angle up or down, and you just dial in."
SECRET SERVICE DIRECTOR: TRUMP RALLY SHOOTER WAS IDENTIFIED AS ‘POTENTIAL PERSON OF SUSPICION’
United States Secret Service Director Kimberly Cheatle conceded that Crooks had been dubbed a "potential person of suspicion" before the assassination attempt.
In an interview with ABC News Tuesday, she said the Secret Service was aware of the security vulnerabilities presented by the building Crooks took a sniper's position on to aim at Trump. However, a decision was made not to place any personnel on the roof.
And Crooks climbed up with an AR-15, took a position and opened fire.
Experts – and casual observers across the board – have said the failure to secure the rooftop with a clear line of sight to the former president was an astonishing security failure.
"It's unbelievable to me," said Sean Parnell, an Army veteran and former congressional candidate from the area who was standing near Trump's stage when gunfire broke out. "My 11-year-old was like, ‘Why is somebody able to come so close to that building?’"
WATCH: Exclusive video shows the aftermath of the assassination attempt on former President Trump
Crooks killed a 50-year-old father of two named Corey Comperatore, critically wounded David Dutch, 57, and James Copenhaver, 74, according to authorities. He struck the former president in his right ear.
"That building in particular has a sloped roof at its highest point. And so, you know, there's a safety factor that would be considered there that we wouldn't want to put somebody up on a sloped roof. And so, you know, the decision was made to secure the building, from inside," Cheatle told ABC.
Secret Service counter-snipers "neutralized" Crooks at the scene, authorities said. Images show him lying dead on a rooftop less than 150 yards from where Trump had been speaking.
OFFICER REPORTED MAN AT TRUMP RALLY WITH RANGE-FINDER 30 MINS BEFORE ASSASSINATION ATTEMPT: SOURCE
Video first obtained by Fox News shows a chaotic scene inside the secure perimeter with members of law enforcement stuck on the wrong side of a metal fence as they tried to reach the sniper's perch.
Sources previously told Fox News Digital that an initial suspicious sighting of Crooks carrying a range-finder before the shooting had been reported to Pennsylvania State Police and "disregarded."
The PSP countered that it had sent dozens of troopers upon the Secret Service's request to assist in rally security – but they were not responsible for covering the building that Crooks used or the property around it. County officials also said they had been initially engaged for traffic patrol assistance and were unaware of who was assigned to control the building.
EXCLUSIVE TRUMP RALLY VIDEO SHOWS CHAOS AFTER GUNMAN OPENED FIRE
Crooks was initially spotted without the rifle about 30 minutes before the shooting, the source said. A local officer thought he was suspicious enough to report. He took a photo, and there was a discussion about whether what he was carrying was a pair of binoculars to try and see the rally better.
Then Crooks reportedly vanished from view – presumably to get his weapon.
The Department of Homeland Security's inspector general has since announced an investigation into the Secret Service's handling of security for the event.
Authorities have said Crooks had no criminal history and no documented mental illnesses. The murder weapon was purchased legally by his father in 2013.
A federal investigation into his motives and potential help is continuing, with the FBI analyzing his laptop, cellphone and rifle as well as interviewing dozens of witnesses.
Searches of the suspect’s house and vehicle wrapped up Sunday night. The bureau also said the FBI had interviewed nearly 100 witnesses, including members of law enforcement, as of Monday afternoon.
Anyone with information on the case is asked to submit tips at http://tips.fbi.gov or 1-800-CALL-FBI.
Fox News' Greg Norman, Audrey Conklin, David Spunt and Jake Gibson contributed to this report.
