Details are beginning to emerge about how the 20-year-old would-be assassin Thomas Matthew Crooks was able to get close enough to Donald Trump at a campaign rally in Butler, Pennsylvania, on Saturday to shoot the former United States president in the ear.

Evading Secret Service agents and local law enforcement officers, Crooks reached a rooftop about 140 metres (450ft) from where Trump was speaking on a stage and fired, grazing the Republican candidate running in the presidential election in November.

Crooks was shot dead by the Secret Service immediately after firing, and an investigation into the incident is ongoing.

In the days since the shooting, digital “bread crumbs” have been gathered via media outlets, photographers at the rally and personal cellphone videos depicting the events leading up to the shooting.

This is the timeline of what is known so far:

Sixty minutes before shooting: Crooks spotted by police

One hour before Trump took to the stage to give his speech, Crooks was spotted by local police officers outside the event perimeter. They believed he was “acting suspiciously”, according to reports. The officers used a radio to alert the Secret Service and other police officers inside the event that a suspicious person had been seen outside the rally perimeter.

In video footage, Crooks can be seen inside the site a short time later. In an exclusive video posted by WTAE-TV, a local ABC news affiliate in Pittsburgh, Pennsylvania, Crooks is shown close to the building where he later fired from about an hour before the shooting.

Forty minutes before shooting: Crooks spotted again with rangefinder

Local police who were trying to keep track of Crooks lost him for roughly 20 minutes before spotting him again.

According to The New York Times, he was carrying a rangefinder, a telescope-like device used to measure the distance from an observer to a target.

It is thought that Crooks was using the rangefinder to measure the distance between the roof and the platform where Trump gave his rally speech.

Twenty minutes before shooting: Crooks spotted on top of roof

Twenty minutes before the shooting, Crooks was spotted by Secret Service on the roof of a complex of interconnected corrugated-metal buildings used by an equipment company, AGR International. Secret Service agents are understood to have reported this to local police and asked them to investigate.

According to The Washington Post, a local police officer was sent to identify the suspicious individual. 

Two minutes before shooting: Rally attendees spot Crooks

Two minutes before the shooting, people attending the rally observed Crooks on the roof. They shouted to nearby police officers that someone was crawling along the roof. By then, a police officer had already been dispatched to investigate.

Thirty seconds before shooting: Officer tries to reach Crooks

Butler County Sheriff Michael T Slupe told the Post that an officer went to examine the roof after a request from local police to try to identify the suspicious individual who had by now been spotted several times.

According to reports, the police officer who had been dispatched managed to lift himself up and grasp the edge of the roof of the building Crooks was on top of to get a look. Crooks then turned and pointed his AR-15-style, semiautomatic rifle at the officer. The officer quickly dropped back down to the ground to avoid being shot.

The shooting and the seconds that followed

Immediately after this, Crooks fired eight shots in Trump’s direction with one grazing Trump’s ear. Three spectators were also hit by bullets – one fatally. He was later named as Corey Comperatore. The two other spectators were critically injured. After Trump ducked down, Secret Service agents formed a protective barrier around him.

A Secret Service sniper team then killed Crooks with a high-powered rifle from the roof of a building behind and off to the side of the stage that Trump was on.

What happens next?

On Wednesday, James Comer, the Republican chairman of the Oversight and Accountability Committee in the US House of Representatives, officially subpoenaed Kimberly Cheatle, the director of the Secret Service, to attend a hearing on Monday. This hearing will be the first in the congressional investigation into the attempted assassination.

Several top Republican leaders, including Senate Minority Leader Mitch McConnell and House Speaker Mike Johnson, have called for Cheatle to resign due to the security lapse on Saturday.

This week, President Joe Biden also ordered an independent review of the security measures in place during the rally.

In an interview with ABC News this week, Cheatle was asked about members of Congress calling for her to resign and stated: “We’re going to continue to be transparent and communicate with people.”

She added: “Absolutely, I do plan to stay on.”