Thomas Crooks was acting strangely. Sometimes he danced around his bedroom late into the night. Other times, he talked to himself with his hands waving around.

These unusual behaviors intensified last summer, after he graduated with high honors from a community college. He also visited a shooting range, grew out his thin brown hair and searched online for “major depressive disorder” and “depression crisis.” His father noticed the shift — mental health problems ran in the family.

On the afternoon of July 13, Mr. Crooks told his parents he was heading to the range and left home with a rifle. Hours later, he mounted a roof at a presidential campaign rally in western Pennsylvania and tried to assassinate Donald J. Trump.

That scene has been etched into American history. After a bullet grazed Mr. Trump’s ear, he lifted his blood-streaked face, pumped his fist and shouted the words: “Fight! Fight! Fight!” Mr. Trump has said that God saved him in order to save America, and the White House recently unveiled a statue in the Oval Office commemorating the moment.

The near miss revealed alarming security lapses that allowed an amateur marksman barely out of his teens to fire at a former president less than 500 feet away. And it galvanized support for Mr. Trump, inspiring voters who saw him as a righteous hero triumphing in the face of smear campaigns, relentless prosecutions and even an attempt on his life.

Now, nearly a year later, with Mr. Trump in his second presidential term, much of the world has forgotten about the 20-year-old who set out to murder him. Mr. Crooks — who also killed a bystander and wounded two others before being shot dead by the Secret Service — had kept to himself and seemed to leave little behind. His motive was a mystery, and remains the source of many conspiracy theories.

A New York Times examination of the last years of the young man’s life found that he went through a gradual and largely hidden transformation, from a meek engineering student critical of political polarization to a focused killer who tried to build bombs. For months he operated in secret, using aliases and encrypted networks, all while showing hints of a mental illness that may have caused his mind to unravel to an extent not previously reported.

This account offers the fullest picture yet of Mr. Crooks’s life. Although many aspects of his background and mental health are still unknown, The Times’s reporting is based on thousands of pages of his school assignments, emails and logs of his internet activity, as well as text messages, government reports and interviews with dozens of people who knew him or were familiar with the case.

Mr. Crooks followed his dark path with seemingly little notice from those closest to him. He stockpiled explosive materials in the small house he shared with his parents in Bethel Park, Pa. When his face was plastered across the news, his classmates couldn’t believe it. Investigators later found a crude homemade bomb inside his bedroom, not far from where his parents slept.

His parents, Matthew and Mary Crooks, did not respond to interview requests, and their lawyers declined to comment. But on the night of the shooting, Matthew Crooks told federal agents that he had been concerned about his son's visits to the gun club.

“I should have known better,” Mr. Crooks said, one of the agents later told congressional investigators.

‘A Really Intelligent Kid’

Before his deadly assault, Thomas Crooks’s only record of trouble was a lunch detention in middle school for chewing gum.

In high school, he earned a top score on the SAT — 1530 out of a possible 1600 — and received perfect marks on three Advanced Placement exams, according to his academic records. He did not socialize much, but came out of his shell in a technology program in which he built computers. His teacher, Xavier Harmon, nicknamed him “Muscles” — an ironic nod to his slight frame — which made him laugh.

One high school classmate said Mr. Crooks enjoyed talking about the economy and cryptocurrencies, encouraging others to invest. On the rare occasions when the conversation turned to politics, he seemed to be in the middle of the road.

On President Joseph R. Biden Jr.’s inauguration day in January 2021, Mr. Crooks donated $15 to a committee backing Democrats. But when he turned 18 that fall, he registered as a Republican. His family’s political affiliations were as diverse as the swing state they lived in: His older sister, Katherine, and his father were registered as Libertarians, and his mother was a Democrat.

Mr. Crooks enrolled in the Community College of Allegheny County. He was the kind of student others sought out for help, and a regular member of a math book club, though he didn’t appear to hang out with friends outside school. He endeared himself to his professors not just with high marks but also for showing up at office hours and trading emails about how to improve his work.

“He seemed like a really intelligent kid — I thought he would be able to do whatever he wanted,” said Trish Thompson, who taught Mr. Crooks engineering. In her class, he designed a chess board for visually impaired people, like his mother.

Mr. Crooks was close with his immediate family, according to a video he recorded in the fall of 2022 for an oral communication class. He described preparing Thanksgiving turkey with his father and baking Christmas cookies with his mother, saying, “I don’t think there’s any better way to spend time with family than cooking meals together.”

Another assignment in that class required him to speak in front of five adults. He asked the professor for an exception, as he had only his parents and possibly his sister. “I do not have access to any other adults,” he wrote.

In April 2023, Mr. Crooks showed a glimpse of his frustration with American politics. In an essay arguing for ranked-choice voting, he lamented “divisive and incendiary campaigns which are pulling the country apart.”

“As we move closer to the 2024 elections we should consider carefully the means by which we elect our officials,” Mr. Crooks wrote. “We need an election system that promotes kindness and cooperation instead of division and anger.”

Around the time he wrote the essay, he began using an alias to buy from online firearms vendors, according to the Federal Bureau of Investigation. He would make at least 25 gun-related purchases before the fateful rally.

Declining Mental Health

Mr. Crooks bought a membership in August 2023 to the Clairton Sportsmen’s Club, a shooting range about 30 minutes from his home. This was not unusual in his community, and his father was a gun enthusiast. By the end of the year, he was visiting the range roughly once a week, including on Christmas Day.

Through a public records request, The Times obtained logs of Mr. Crooks’s internet activity while he was signed in to the community college network. The records are somewhat limited: They show website domains rather than specific pages, and Mr. Crooks often used an encrypted connection to obscure his online footprint.

Still, the digital trail suggests that he was focused on Mr. Trump, the news and guns.

On Dec. 6, 2023, about seven months before the shooting, he rapidly cycled through about a dozen news websites, including CNN, The New York Times and Fox News, before visiting the Trump administration’s archives, the logs show. Minutes later, he visited seven gun websites, including one focused on the AR-15, similar to the rifle he would use in the attack. Later that day, he paid a visit to the shooting range.

The next month, he placed a $101.91 order online for more than two gallons of nitromethane, a fuel additive that can be used in explosives, giving his home address for delivery. The package did not arrive promptly.

“I have not received any updates of the order shipping out yet,” he wrote to the seller on Jan. 31, 2024. He used his community college email account, but included a screenshot of his order confirmation showing he had provided an encrypted email address. “I was wondering if you still have it and when I can expect it to come.”

On Feb. 26, a couple of hours before a physics class, he visited a series of websites, including an ammunition manufacturer, the Trump campaign site and NBC News, as well as YouTube, Reddit, Spotify and a site for Xbox users.

Interviews with his teachers, friends and co-workers suggest that many people who interacted with him regularly did not know he was troubled, let alone capable of premeditated murder.

He had worked for years as a part-time dietary aide at the Bethel Park Skilled Nursing and Rehabilitation Center. Employees said Mr. Crooks was punctual and dependable, though he didn’t talk much. He showed up for work in the weeks before the shooting and nothing seemed amiss.

“What I heard from people in his department is that there was no clear indications of changes in his behavior or routine,” said Reggie Brown, a former human resources manager at the center.

After back-to-back semesters on the dean’s list, he earned his associate degree in engineering and was set to transfer to Robert Morris University. He had told classmates he hoped to have a career in aerospace or robotics.

His father noticed his mental health declining in the year before the shooting, and particularly in the months after graduation. He later told investigators that he had seen his son talking to himself and dancing around his bedroom late at night, and that his family had a history of mental health and addiction issues, according to a report from the Pennsylvania State Police, parts of which were shared with The Times. The younger Mr. Crooks was also making the depression-related queries online, investigators found.

Representative Clay Higgins, a Republican from Louisiana who worked on a congressional task force on the shooting, told The Times that he learned worrisome information about Mr. Crooks’s mental health while investigating the case on a trip to Pennsylvania.

He was “having conversations with someone that wasn’t there,” Mr. Higgins said, adding that many questions remained unanswered. “There was a mysteriousness to Thomas Crooks’s descent into madness.”

In the final month before the shooting, Mr. Crooks conducted more than 60 searches related to Mr. Trump and Mr. Biden, the F.B.I. said. And yet there were hints that he hadn’t fully committed to an attack.

“When can I expect the diploma to be mailed?” he wrote to his college registrar.

About a week before the shooting, Mr. Crooks’s internet searches became especially focused, the F.B.I. said. On July 6, he registered for Mr. Trump’s rally at the fairgrounds in Butler, Pa., and searched, “How far was Oswald from Kennedy?” In his remaining days, he looked up where Trump would be speaking on the site.

Just after 6 p.m. on July 13, Mr. Crooks fired eight bullets toward Mr. Trump. Investigators later found two explosive devices in the trunk of the car that he had driven there.

As word spread the next day that he was the gunman, one of his few friends from community college reached out.

“Hey Thomas, you weren’t the person who tried to shoot Trump and then got killed right?” texted the friend, who was interviewed by The Times but requested anonymity because he feared being associated with Mr. Crooks. “I really liked you as a friend and I desperately don’t want you to be dead.”

A Homemade Explosive

Shortly before 11 p.m. on the night of the shooting, Mr. Crooks’s father called 911, saying he had not seen his son since that afternoon.

“We’ve gotten no contact from him, no text messages, nothing’s been returned, and he’s not home yet,” Matthew Crooks told the operator.

“That’s totally not like him,” he added. “So we’re kind of worried, not really sure what we should do.”

Agents with the Bureau of Alcohol, Tobacco, Firearms and Explosives had begun surveilling the 1,000-square-foot house, according to a transcript of agent testimony provided to Congress.

Around 11:40 p.m., the agents approached the house. Matthew Crooks opened the screen door and asked, “Is it true?”

They entered and noticed Mary Crooks sitting with the family cat in the living room, television on. The agents swept the house for potential dangers.

Down a short hallway was the young Mr. Crooks’s bedroom, door open. The room was fairly organized, with a made bed and large 3-D printer. An empty pistol belt and holster lay on the floor.

At the room’s threshold, an agent looked down and saw a .50-caliber, military-grade ammunition can “with a white wire coming out,” according to the testimony. The agent also observed a gallon jug labeled “nitromethane” in the closet.

The agents immediately evacuated. While they waited for the bomb squad, they interviewed Mr. and Ms. Crooks outside late into the night, asking about their family and what made their son “tick.”

The parents were calm and polite. They said Thomas loved building things, like computers, and visiting the gun range. They didn’t think he had any friends or girlfriends. His father said he didn’t “know anything” about his son, according to the testimony.

On the subject of politics, Matthew Crooks said his son would “go back and forth and kind of argue both sides,” an agent testified. The father said Thomas would talk about Mr. Trump and Mr. Biden, but “never really indicated that he liked one or the other more.”

Mary Crooks, who had been mostly quiet, spoke up to ask if her son was really dead. The agent told her yes, and she began to cry. Her husband “put his hand out and said, you know, ‘It’s OK. It’s not true until we see the body,’” the agent testified.

The couple has kept a low profile since the shooting. This spring, Ms. Crooks left the job she had held for 27 years — as a rehabilitation therapist for the visually impaired at a state agency — because of the shooting, according to a resignation letter obtained by The Times. “Certain circumstances have left me with no other option,” she wrote.

Matthew Crooks had been in social services for over two decades, first working with spina bifida patients and later managing the medical care of patients in a Pittsburgh health system. The health system declined to say whether he still worked there.

Madeleine Frizzi, the mother of Ms. Crooks, was short when asked about her daughter and son-in-law. “I do not have any contact with them — whatsoever,” she said, declining to elaborate.

A Cloud of Conspiracy Theories

The F.B.I. has led the investigation into Mr. Crooks, working with the A.T.F. and the Pennsylvania State Police. In the weeks after the shooting, the F.B.I. released preliminary findings based on details gleaned from interviews and Mr. Crooks’s devices suggesting that he had been planning an attack for over a year.

In a news conference late last July, F.B.I. officials said they had not found evidence of mental health treatment, institutionalization or medications. The next month, the agency said Mr. Crooks had begun searching online about how to make explosives as early as 2019, when he was 16, but did not elaborate on the timeline. Investigators said they had not uncovered a motive or any co-conspirators.

In the absence of new information, conspiracy theories about Mr. Crooks have grown. Some have claimed that he had an accomplice, or that he was an agent of the so-called deep state. Kelly Little, who lives across the street from the Crooks house, said that another theory floating around claimed that she and other neighbors had built underground tunnels to aid the shooter.

“Why do we still know nothing about that guy in Butler?” Elon Musk asked in February in front of a large crowd at the Conservative Political Action Conference outside Washington. “Kash is going to get to the bottom of it,” he added, referring to Mr. Trump’s F.B.I. director, Kash Patel. The crowd cheered.

But in a recent interview on Fox News, Mr. Patel at his side, Dan Bongino, the deputy director of the F.B.I., said there was simply no “big, explosive there there.” He added, “If it was there, we would have told you.”

Helen Comperatore, an avid Trump supporter whose husband, Corey, was killed by Mr. Crooks, still wants to know more. She told The Times she had not received any official updates from investigators in months and felt she was owed a fuller explanation of what had happened.

“I am praying the president gets to the bottom of it and keeps working on this case for me — and him,” she said.

Reporting was contributed by Emily Cochrane, Adam Goldman, Steven Rich, Glenn Thrush and Aric Toler. Julie Tate contributed research. Produced by Gabriel Gianordoli, Jenni Lee and Rumsey Taylor.