Just before a gunman opened fire on former President Donald Trump at a rally in Butler, Pa., law enforcement was on the verge of a major breakthrough. They had spotted the suspicious individual, Thomas Matthew Crooks, acting oddly but lost track of him. Despite having a clear shot, Crooks managed to climb onto a warehouse roof outside the security perimeter, from where he fired at Trump and killed a rally attendee. The Secret Service, who had been warned about Crooks but didn’t act on it, now faces intense scrutiny for failing to secure the area properly and for the series of missed opportunities that led to the shooting.

Butler, Pennsylvania, police enforcement was about to pull off a major policing coup, about an hour before a shooter opened fire, almost killing a former president.

At a campaign rally on Saturday, thousands of people flocked to support former President Donald J. Trump. Local police noticed one slender young guy acting strangely and alerted other law officials. Through radio contact, the Secret Service was also notified. It didn’t look like the suspicious individual was carrying a weapon reports The New York Times.

Amazingly, police had located the proper man: Thomas Matthew Crooks, a potential assassin who was unknown to them at the time. Then they were unable to find him.

Twenty minutes before the carnage started, Mr. Crooks was once more seen by a sharpshooter who took his image from a distance.

At least two local officers were taken off traffic duty to assist in the man’s search as time passed. However, Mr. Trump was able to take the stage without interference from the Secret Service, the organization tasked with his protection. Mr. Crooks opened fire eight minutes after Mr. Trump began speaking, leaving the Republican presidential contender wounded and a rallygoer dead.

Several Secret Service actions, including the decision to allow the rally to go as law enforcement searched for a person who might be violent, are currently being questioned. The agency is also being questioned for permitting the exclusion of a structure that was within the rifle’s firing range from its tight perimeter, which allowed the gunman to take advantage of a blind spot near the former president.

Following a private briefing with the F.B.I. and Secret Service, Tennessee Senator Marsha Blackburn, a Republican, posted on X on Wednesday, saying, “I am appalled to learn that the Secret Service knew about a threat before President Trump walking onstage.”

There are several investigations into the errors in judgment, one of which was made public by President Biden on Sunday. Alejandro N. Mayorkas, the secretary of homeland security, described the incident, which left one rally attendee dead and two others hospitalized, as a security lapse, but he maintained his support for Secret Service Director Kimberly A. Cheatle.

In an interview with ABC News, Ms. Cheatle stated that she accepts full responsibility.

Investigations into what transpired are still ongoing, but it is evident that numerous chances to stop Mr. Crooks before things got lethal were lost. This story is based on statements made by several federal authorities, Butler, Pennsylvania law enforcement personnel, and members of Congress who received briefings from the FBI and Secret Service. It also includes video footage from the rally.

An advance crew inspected the Butler Farm Show grounds on July 8 in order to identify any potential security risks. Working with local law enforcement, agents clarified what the Secret Service would manage and what was expected of them. Crucially, even though the warehouses were barely 450 feet from Mr. Trump’s podium, the Secret Service decided to keep a set of buildings north of the stage out of the security zone. That was reachable with a rifle.

This meant that local law enforcement was tasked with securing the warehouses. The warehouse building located north of the protest site was used as a surveillance station by the local police and Secret Service. It was seen as a location to observe Mr. Trump’s entourage rather than as something that required constant observation.

However, that left a blind area that was outside the security cordon but still well inside Mr. Trump’s rifle range. It was taken advantage of by a gunman who lacked tact and military experience. He arrived early and behaved strangely enough for the police to take his picture and distribute it, even though he was not carrying a weapon.

“I don’t know whose responsibility that building was,” Butler County District Attorney Richard Goldinger said. “But somebody should have been there.”

Nor was it evident how far forward Mr. Crooks had planned. Mr. Trump made public his July 3 event in Butler.

However, after Mr. Crooks’s smartphones and other electronic devices were ultimately accessed by the FBI, officials discovered that he had looked up pictures of Mr. Trump, President Biden, Attorney General Merrick B. Garland, and even FBI Director Christopher A. Wray.

Additionally, Mr. Crooks entered “major depressive disorder” and looked up the locations and dates of Mr. Biden’s and Mr. Trump’s performances.

Mr. Crooks’s home in Bethel Park, Pennsylvania, was approximately fifty miles away from one of Mr. Trump’s scheduled appearances.

According to an FBI investigative summary, Mr. Crooks visited a gun range on Friday, July 12, the day before the attempted assassination. According to the F.B.I. document and federal law enforcement officials, he acquired a ladder at a Home Depot the following morning and 50 rounds of ammunition from a gun shop close to his house later that day.

Mr. Crooks packed an AR-15-style gun that his father had purchased more than ten years prior in his Hyundai Sonata. And he brought two homemade bombs, built from empty ammunition cans about the size of a toolbox, packed with a potentially deadly mixture of fuel and fertilizer.

As per a separate federal government investigation obtained by The Times, the devices were equipped with a remote-control receiver, which is the same kind of device usually used to remotely trigger fireworks displays. According to the assessment, the devices seemed to be made with a remote control in mind. He also brought that.

The Butler event for Mr. Trump was scheduled to begin at 5 p.m., but he did not take the stage until an hour later. WTAE-TV in Pittsburgh was able to collect video, which seems to indicate that Mr. Crooks arrived by at least 5:06 p.m.

According to the footage, Mr. Crooks was moving around in front of the warehouse that he would later utilize as a location for a sniper’s nest. With a hand in his pocket, he appeared unarmed and leisurely, gazing toward the rally venue.

At the demonstration, the cop took a picture of him and shared it with other officers. According to the official, local police attempted to pursue the dubious man but were unable to do so.

The shooting didn’t happen for approximately twenty minutes.

AGR International is an equipment company, and Mr. Crooks eventually ascended to the roof of building No. 6, one of a series of connected corrugated metal warehouses.

For a would-be sniper, the site offered evident advantages: an elevated and unobstructed view of the platform where Mr. Trump was scheduled to speak. According to Ms. Cheatle, the slope of the roof raised safety issues, thus no officers were posted on the actual roof.

Different people had different ideas on how Mr. Crooks climbed to the roof. According to a Secret Service spokesman on Wednesday, he had independently ascended to the top, maybe with the help of an air conditioner. Even though Mr. Crooks had purchased a ladder that morning, federal investigators think he did not use one.

Mr. Trump emerged at 6:03 p.m. and waved to the applauding audience. Six minutes later, witnesses saw Mr. Crooks crawling into position on the roof while Trump, now speaking animatedly onstage, continued to talk. They informed the neighborhood police, who were keeping an eye on the area outside the Secret Service’s boundary.

Two minutes had passed before the gunshots.

Appearing oblivious, Mr. Trump carried on with his speech on stage.

However, the Secret Service unit surrounding him started to react frantically, turning its attention from surveying the throng to the area north of the protective fence. Everyone’s attention was now on the large warehouse complex immediately to the north, also known as the blind spot.

A counter-sniper squad from the Secret Service swiftly climbed from one side of the peaked roofs to the other on a barn immediately behind Mr. Trump. They now aimed their weapons at the warehouse, which is located roughly 450 feet to Mr. Trump’s right.

It’s not apparent if the Secret Service counter-snipers were able to see Mr. Crooks, who was on the roof of the warehouse. The gentle rise of the warehouse roof most likely obscured these snipers’ perspective, according to a visual analysis by the New York Times.

Mr. Crooks was low-crawling up the other side, remaining hidden.

Traffic around the warehouse was being directed by officers from the local Butler Township Police Department, who were stationed there. Butler Township Commissioner Edward Natali said on social media that two of the officers—at least—left their traffic posts to assist in the search for the suspicious individual.

When the two officers arrived at the warehouse, one of them raised the other until his head was above the ceiling. He saw Mr. Crooks, and he saw him. Mr. Crooks “turned his firearm,” according to Mr. Natali, but the officer was clinging to the roof with both hands, making it impossible for him to shoot back.

According to what Mr. Natali wrote, the officer was hurt when he fell backward.

Mr. Crooks made it to the highest point on the warehouse roof, where he could see over the edge. A witness on the ground yelled: “He’s on the roof! He’s got a gun!”

The allotted time was then up. An investigation of the audio from the site by the Times revealed that Mr. Crooks fired his gun eight times. His initial shot seemed to miss Mr. Trump, causing his right ear to bleed. In addition to Corey Comperatore, a 50-year-old retired firefighter, two other rally attendees were hurt.

Then he was shot once by a Secret Service sniper in the south barn. The Butler County district attorney stated that although a local police officer in another part of the region also fired at him, it was unclear if his bullet hit Mr. Crooks.

Mr. Crooks was found dead on the roof without any identification when the police arrived. Officers were able to link his father to the serial number on his firearm. He had a remote control for the bombs in his automobile in his pocket.

It was unclear if he had attempted to use it or if the bombs were constructed sufficiently to go off.