Thomas Matthew Crooks began searching for terms related to bomb-making when he was 15. Five years later, he had two homemade explosive devices in his car when he drove to Butler County, Pennsylvania, to assassinate former President Donald Trump on July 13.

It is unclear how — or even if — Crooks planned to use the explosive devices. He never detonated them before using an AR-15 rifle to fire eight rounds at Mr. Trump before a Secret Service sniper shot and killed him.

Why Crooks built bombs he did not use is among the many mysteries that linger nearly two months after the assassination attempt. But the biggest unanswered question is Crooks’ motive for the attack in which a bullet grazed Mr. Trump’s right ear.

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Another unknown is whether Crooks intended to harm anyone besides Mr. Trump or if others just got caught in the crossfire. 

One rallygoer, 50-year-old firefighter Corey Comperatore, was killed. Two others, 57-year-old David Dutch and 74-year-old James Copenhaver, were injured.

FBI investigators believe Crooks acted alone. The bureau said it had conducted nearly 1,000 interviews in its ongoing investigation and analyzed hundreds of hours of video footage.

The FBI described Crooks as a loner who had few associates outside of his family. His online search history shows a mix of ideologies and revealed that he looked at several potential targets, searching for information on events for President Biden and Mr. Trump, as well as the Republican and Democratic national conventions. 

Honing in on Trump Rally

After Mr. Trump announced his plans to hold a July 13 rally at the Butler Farm Show grounds in western Pennsylvania, Crooks “became hyper-focused on that specific event and looked at it as a target of opportunity,” Kevin Rojek, special agent in charge of the FBI Pittsburgh Field Office, said. 

Crooks, who lived roughly an hour south of Butler in Bethel Park, registered to attend the rally on July 6. That same day he searched how far Lee Harvey Oswald was from President John F. Kennedy when he assassinated him and “where will Trump speak from at Butler Farm Show.”

The next day, on July 7, Crooks traveled to the rally site and spent about 20 minutes surveying the area. In the following days, he searched for “AGR International,” a glass and plastics manufacturer whose headquarters is located next to the Farm Show grounds, “ballistic calculator” and “weather in Butler.” 

The day before the rally, Crooks visited a local shooting range and practiced with the AR-15 he purchased from his father in October. 

Crooks bought 50 rounds of ammunition at a local gun store the day of the rally, which the FBI said were among those he fired that day.

The morning of July 13, Crooks spent over an hour in the area of the rally, then traveled home and obtained a rifle around 1:30 p.m., telling his parents he was heading to a nearby shooting range.

Crooks arrived back at the rally site in the late afternoon and flew a drone from about 200 yards out between 3:51 p.m. and 4:02 p.m. The FBI said the drone’s flight path indicates its aerial views would have helped Crooks assess the security posture at the event.

Crooks never went into the event. Instead, he spent time wandering the adjacent AGR complex, outside of the security perimeter. 

It was there that local police flagged him as a suspicious person after spotting him using a rangefinder roughly half an hour before the rally. But law enforcement lost sight of Crooks and was unable to find him to question him. 

At 6:05 p.m. Crooks used HVAC equipment and a pipe to climb onto the roof of the AGR building closest to the stage. Laying on the back portion of the sloped roof, Crooks fired eight rounds toward the stage at 6:11 p.m. 

Crooks was killed by a Secret Service sniper, so he is not around to answer for his crimes.

When police recovered his body, they found a remote detonator. The FBI later said the receivers on the explosive devices in his car were in the off position but otherwise capable of exploding. A separate explosive device found in his home was missing initiation system components and was non-functional. 

Gun Enthusiast

Questions remain about Crooks’ firearm training and how he was able to get within inches of killing Mr. Trump from roughly 130 yards away. But he clearly had an interest in guns.

The Clairton Sportsmen’s Club, which has a 200-yard-rifle range, confirmed Mr. Crooks had been a member. 

On the day he sought to assassinate Mr. Trump, Crooks was wearing a T-shirt from Demolition Ranch, a firearms-centered YouTube channel. Matt Carriker, the channel’s Texas-based creator, said in a video that he was “shocked and confused,” but noted he does not vet the people who buy his shirts. 

“This channel was never meant to incite violence or hate,” he said. 

The FBI found that Crooks, starting in the spring of 2023, made more than 25 firearms-related purchases from an online vendor using an alias. 

Crooks’ family, his parents in particular, have been “extremely cooperative” in the FBI investigation, providing all information asked of them, Mr. Rojek said. 

His parents have not been speaking to the news media.

Online Activity

While the FBI is “not ready to make any conclusive statements regarding motive,” analyzing Crooks’ online search activity helped provide “a clear idea of mindset,” Mr. Rojek said. 

Crooks conducted multiple searches related to explosive devices, looking into bomb-making as far back as September 2019 and continuing through this summer. His search terms included: “detonating cord;” “blasting cap;” “how to make a bomb from fertilizer;” and “how to remote detonators work.”

Crooks also searched for ammonium nitrate, nitromethane and other materials used to manufacture explosive devices. The components used in the explosive devices found in Crooks’ car were legal and readily available online, the FBI said. They found six online purchases Crooks made, using aliases, of chemical-related materials used to create explosive devices.

FBI Deputy Director Paul Abbate said Crooks’ parents did not view his chemical purchases as a reason to believe he was planning an attack. “It’s our understanding from the parents and others that the shooter had a long interest in science and things like this and had been doing experiments and things like this,” he said.

The New York Times reported that Crooks, in a statement for his induction into the National Technical Honor Society as a high-school junior, said he had a lifelong interest in building things, and that he and his father had built a computer together in 2017. 

The FBI found that Crooks searched for Mr. Trump’s campaign schedule as far back as late September 2023 but that from April to July of this year he searched events for both Mr. Trump and Mr. Biden — with more than 60 searches conducted in the 30 days before the attack.

Crooks used a couple of foreign-based encrypted email accounts, which the FBI initially had trouble accessing. Once the investigators got in, they found the accounts were primarily used to purchase firearm components, chemicals and other explosive components.

Few Political Views 

Little and conflicting information is available about Crooks’ political views. He registered as a Republican in 2021, when he turned 18.

Earlier that year, on Mr. Biden’s Inauguration Day, Crooks donated $15 to the Progressive Turnout Project, a Democratic-aligned political action committee. A spokesperson for the group told CNN the donation came from an email campaign about tuning into the inauguration and that it was the only contribution associated with that email address, which was unsubscribed from its lists two years ago.

The FBI found a social media account associated with Crooks containing more than 700 posts from 2019-2020, some of which reflect antisemitic and anti-immigration themes and espoused political violence. The FBI in late August was still working to determine whether Crooks, who would have been in his mid-teens at the time, authored those posts.

While the FBI did not specify which site those posts were found on, the CEO of Gab said his company turned over records for an account with the username “EpicMicrowave” believed to be Crooks’ that showed different political leanings.

“To the best of Gab’s knowledge, as of 2021, Crooks was a pro-lockdown, pro-immigration, left-wing Joe Biden supporter,” CEO Andrew Torba posted on X.

Mental State

A lot of questions remain about Crooks’ mental state. His parents are both licensed counselors, and his father worked at a local behavioral health provider, according to the New York Times.

The outlet separately reported that Crooks looked up “major depressive disorder” on a cellphone found at his house. The FBI has not ruled out mental health issues but said on a July 29 media call that it had no information or evidence of mental health treatment, institutionalization, medications or anything confirming any disorders. 

The FBI and other law enforcement officials reportedly told Mr. Trump during his witness interview that Crooks may have been struggling for years with an undiagnosed disorder.

Citing interviews with Crooks’ family and others who knew him, investigators told Mr. Trump that throughout high school, Crooks would routinely sway back and forth while standing at the bus stop but never received any sort of formal diagnosis related to it, according to ABC News.

Investigators also told Mr. Trump that Crooks was strikingly intelligent, saying he scored higher than 1500 on his SAT and could name every U.S. president.

Rep. Mike Kelly, the Pennsylvania Republican who chairs the House task force on the assassination attempt, told Punchbowl News that the FBI said Crooks started exhibiting strange behavior in the days before July 13.

Crooks was “walking around the house talking to himself” and “flapping his arms,” Mr. Kelly said. “There was some indication that something was wrong.”

‘A Lot of Care’

Crooks graduated from the Community College of Allegheny County in May with a two-year associate’s degree in engineering. He planned to attend Robert Morris University, a private, four-year university located outside of Pittsburgh.

One of his community college instructors, who declined to be identified, told Reuters that Crooks was polite and went “above and beyond” in school. She recalled an assignment to redesign a toy for people with disabilities in which Crooks used a 3D printer to create a chess set for the blind. 

“He put the Braille on it. He talked to experts in the field,” the instructor said. “He really took a lot of care.”

That level of care was also noticed by one of Crooks’ coworkers at Bethel Park Skilled Nursing and Rehabilitation Center, who told CNN he was “the sweetest guy.” 

The coworker, who declined to be named, described working with Crooks, who was a dietary aide, in the week before the shooting to find an easier way for nursing home residents to open ranch dressing packets in providing an example of how caring he was. 

Crooks never expressed political views at work and wasn’t “a radical,” the coworker said, adding, “It’s hard seeing everything that’s going on online because he was a really, really good person that did a really bad thing, and I just wish I knew why.”

The coworker also went to Bethel Park High School with Crooks, who graduated in 2022 and won a $500 award for math and science in the school’s annual recognition program that year. 

Bullied or Not?

Former high school classmates who spoke to various news outlets described Crooks mostly as a quiet, loner. But there are disparate accounts as to whether he was bullied in school. 

Jason Kohler said Crooks had “no facial expression” when he walked through the school hallways and was bullied by other students. “He wasn’t, like, with the clique, so he always had, I guess, a target on his back,” he told CNN. 

Another former classmate, who asked not to be named, said Crooks was shy but he had a group of friends who were fairly conservative, some of whom would wear Trump hats. “I just couldn’t believe he did something that bold, considering he was such a quiet and kept-to-himself kind of person,” she said of his attempt to assassinate Mr. Trump.

Mark Sigafoos, who shared two classes with Crooks, recalled him as quiet but “very kind,” telling The Philadelphia Inquirer: “He was a nerdy kid, but I don’t think he was as harshly bullied as some people are saying.”

Mr. Sigafoos did not recall him making political overtures in class, but Max R. Smith told the newspaper that Crooks “definitely was conservative” based on his experience with him in a sophomore American history course.

Mr. Smith cited a mock debate in which students had to pick a side of the classroom to stand on, signaling their support or opposition to a given proposal.

“The majority of the class were on the liberal side, but Tom, no matter what, always stood his ground on the conservative side,” he said.

The Bethel Park School District said in a statement that the notion that Crooks was relentlessly bullied in school is “a painful misconception,” saying they keep detailed records that show he “excelled academically, regularly attended school, and had no disciplinary incidents, including those related to bullying or threats.”

The school district also said it had no records of Crooks ever trying out for or being rejected from the high school’s rifle team, as some students claimed. “It is possible that the shooter informally attended a practice, took a shot, and never returned,” the district said.