Police had clues that something was amiss before the Trump rally shooting

So far, details about the man who authorities say tried to kill former President Donald Trump amount to little more than a stick-figure drawing: a politically enigmatic loner who worked in a nursing home.

Still absent are the motivations behind the assassination attempt at Saturday's rally in Pennsylvania.

That's not all. Investigators are still trying to piece together how Thomas Matthew Crooks eluded Secret Service agents and local police.

Officers and spectators alike took notice of Crooks before the shooting. And the roof of the building that he fired from appeared to be unsecured, leaving a security expert baffled.

Here's what we know and don't know about the shooting.

What we don't know

The shooter's motive: Authorities have conducted hundreds of interviews, cracked into the shooter's phone and searched his car and home. A motive behind the attempt on Trump's life remains unknown.

Why wasn't he stopped: Police had a report of a suspicious man at the rally. And witnesses pointed at an armed man on a nearby roof that a police officer climbed onto. Crooks still fired several shots that killed one spectator, wounded two others and grazed the former president's ear.

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Why the building was unsecured: The would-be assassin fired his AR-style rifle from a building that was close enough for a marksman to hit a human-sized target. A security expert said the structure should have been secured and under surveillance.

What we know

Police were aware of Crooks at the rally

Spectators at the Trump rally noticed a man later identified as Crooks pacing outside the event's metal detectors. Police had reports of his behaviour and were apparently exchanging photos of the suspect. Witnesses later pointed and shouted about the man with a rifle on the roof.

Butler County Sheriff Michael Slupe told The Associated Press that a local officer climbed to the roof and encountered Crooks, who saw the officer and turned toward him just before the officer dropped down to safety.

Crooks quickly took a shot toward Trump. That's when Secret Service snipers shot Crooks, according to two officials who spoke to the AP on condition of anonymity to discuss an ongoing investigation.

Slupe said the officer could not have wielded his own gun under the circumstances. Butler Township Manager Tom Knights added that the officer lost his grip and was not retreating when he fell and severely injured an ankle.

Still, Stan Kephart, a former police chief who worked event security for two former presidents, said the shooting followed "an absolute and abysmal failure" on the part of the Secret Service to protect Trump. The agency is ultimately responsible for the candidate's safety, he added.

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Building's roof was in striking distance

Crooks was an estimated 147 yards (135 metres) from where Trump was speaking, a distance from which a decent marksman could reasonably hit a human-sized target. That is a distance at which U.S. Army recruits must hit a scaled human-sized silhouette to qualify with the M-16 rifle.

Patrick Brosnan, a former New York Police Department detective who runs a national private intelligence and security firm that has protected heads of state, suggested a building that close to the stage should have been secured and under surveillance from the start.

"It would just seem to me to be elemental and rudimentary as it relates to providing a steel band or circular band of protection for the former president, who is fully exposed," said Brosnan, who has trained with the Secret Service and spoken in the past to Trump and his family about security issues.

FBI believes Crooks acted alone

The FBI believes Crooks, who had bomb-making materials in the car he drove to the rally, acted alone with a gun purchased by his father. But the agency has not identified a clear ideological motive.

The FBI said Monday that analysts had gained access to the shooter's phone, though a law-enforcement official who was not authorized to discuss the matter publicly and spoke on condition of anonymity said the device has not revealed meaningful information about any potential motive.

Crooks worked at a nursing home as a dietary aide, a job that generally involves food preparation. Records show that he was registered as a Republican voter in Pennsylvania. But federal campaign finance reports also show he gave US$15 to a progressive political action committee on Jan. 20, 2021, the day Democratic President Joe Biden was sworn into office.

Jason Kohler, who said he attended the same high school but did not share any classes with Crooks, said Crooks was bullied at school and sat alone at lunch time. Other students mocked him for the clothes he wore, which included hunting outfits, Kohler said.

"He was bullied almost every day," Kohler told reporters. "He was just a outcast."