Mr. Saipov drove a semi-truck for a living, logging tens of thousands of miles back and forth across the country, from Denver to Detroit, from Canton, Mass., to Salt Lake City. He moved his wife and children from state to state, always searching for something — friends in Ohio, a new life in Florida, family in New Jersey, where he started driving for Uber six months ago. Nothing ever stuck.

And on Tuesday, Mr. Saipov, now 29, who had spent so many hours on the road by himself, who a former friend said had “monsters inside,” decided to drive one last truck, this one a Home Depot rental, down a crowded bike path on the West Side of Manhattan, the authorities said. Eight people died.

As with any attack like this, there is no single reason Mr. Saipov reportedly decided to kill innocents, mostly tourists enjoying a blustery fall day, 56 degrees with blue skies. He had come to the United States as a moderate Muslim with dreams of making it. He married another Uzbek immigrant and fathered three children. But life did not work out the way Mr. Saipov had wanted. He could not find a job in the hotel business, in which he had worked back home. He developed a violent temper. He lost jobs. An imam in Florida worried that Mr. Saipov increasingly misinterpreted Islam.

“I used to tell him: ‘Hey, you are too much emotional. Read books more. Learn your religion first,’” said Abdul, the imam, who did not want his last name used because he feared reprisals. “He did not learn religion properly. That’s the main disease in the Muslim community.”

As with any attack like this, there is no single reason Mr. Saipov reportedly decided to kill innocents, mostly tourists enjoying a blustery fall day, 56 degrees with blue skies. He had come to the United States as a moderate Muslim with dreams of making it. He married another Uzbek immigrant and fathered three children. But life did not work out the way Mr. Saipov had wanted. He could not find a job in the hotel business, in which he had worked back home. He developed a violent temper. He lost jobs. An imam in Florida worried that Mr. Saipov increasingly misinterpreted Islam.

“I used to tell him: ‘Hey, you are too much emotional. Read books more. Learn your religion first,’” said Abdul, the imam, who did not want his last name used because he feared reprisals. “He did not learn religion properly. That’s the main disease in the Muslim community.”

In Tashkent, Mr. Saipov grew up in a well-off family who practiced traditional Islam and never embraced extremism, the Uzbek government said on Wednesday. His neighbors there said Mr. Saipov never raised suspicions and “always carried himself in a measured and friendly way,” according to the government statement. He never crossed paths with the police.

From 2005 to 2009, Mr. Saipov studied at the Tashkent Financial Institute, one of the biggest universities in Uzbekistan, a former Soviet republic in Central Asia that at the time was run by Islam Karimov, an autocrat called one of the most oppressive leaders in the world. After college, Mr. Saipov worked as an accountant for the Sayokhat hotel in Tashkent, described as having dated rooms and indifferent customer service.

Then he won the green card lottery, which meant he was one of the lucky ones who could escape the repression of home and immigrate legally to America. In March 2010, Mr. Saipov flew into Kennedy International Airport in New York City, his first time in the United States, and made his way to the home of a friend of his father’s in the northeast outskirts of Cincinnati. He skipped the mandate of his government to register with the consular. There were no plans to go back.

Mr. Saipov wanted to secure a job in the hotel business. But with little English and no connections, he had no luck finding work. His father’s friend, a trucker, asked him to move out and find his own place because he wasn’t earning any money. So Mr. Saipov found his first job: as a trucker. And by the end of 2011 he had found his own place, an apartment 200 miles away, in Cuyahoga Falls, Ohio, on a street called Americana Drive.

He wasn’t exactly an extremist. Mr. Saipov liked fancy clothes, a vanity frowned on in conservative Islamic circles. He cursed as if he couldn’t help it. He routinely showed up late for Friday prayers at the Islamic Society of Akron and Kent. He displayed only rudimentary knowledge of the Quran.

Over the three years he lived in the area, he started to change, said Mirrakhmat Muminov, a truck driver and local community activist. He became argumentative, aggressive even, and started to grow out his beard. Mr. Muminov described him as someone “with monsters inside.”

“I always thought deep in my soul that he would be jailed for beating someone or insulting someone,” Mr. Muminov recalled. “He had a vulgar character.”

Although his grandfather sometimes visited from Uzbekistan, his parents never did, Mr. Muminov said. Mr. Saipov made his own family: In 2013 he married an Uzbek immigrant almost six years younger, Nozima Odilova, who ended up in Ohio after landing first in Las Vegas. They started a family, first a daughter, and then a second. He told friends that he really hoped for a son.

While in Ohio, Mr. Saipov formed two businesses as part of his fledgling truck career — Sayf Motors, a play on his name, launched only 14 months after he arrived in the country, and a more Midwestern-sounding company, Bright Auto. But Mr. Saipov mainly drove for others, companies like Abror Logistics out of Paterson, N.J.

Over the years, he racked up at least nine tickets, in Iowa, Missouri, Nebraska, Pennsylvania and Maryland. Those tickets are the closest thing to snapshots of how Mr. Saipov lived on the road.

There he was in Iowa, in December 2011, waiting for 35 minutes along Interstate 80 as officers checked his truck and documents, wrote him a ticket and let him go on his way to Salt Lake City. There he was in Iowa again, in April 2014, stopped for more than an hour for having a cracked windshield and for missing a reflective device while driving a load of cars from Denver to Detroit.

And there he was repeatedly at the weigh station at the 415-mile marker on Interstate 80 in Nebraska. Mr. Saipov received a ticket there for driving too long without required rest and for carrying a load just slightly more than allowed.

By the end of 2015, Mr. Saipov had moved to Tampa, Fla., chasing something. His problems deepened, Mr. Muminov said. He had a hard time finding work. He ran out of money. He exploded in anger.

“He had a character problem,” said Abdul, the imam.

He became more obsessed with the physical trappings of Islam: the long beard, the ankle-high pants. He never spoke of violence, though.

By March last year, he had found a new job, with IIK Transport out of Illinois. A company official, who asked not to be identified because he did not want to be associated with someone accused of terrorism, described Mr. Saipov as someone who “worked the lower 48” states and was “just a regular guy.”

That was when he had his biggest run-in with the law, after a ticket went unpaid and turned into a warrant. It caught up with him at a weigh station in Missouri. He spent 40 minutes on Oct. 20, 2016, at the St. Charles County jail, where he posted a $200 bond with his credit card.

Over the years, he made little impression except for those tickets. “Don’t remember him,” said Jim Klepper, a lawyer who represented him on a ticket in Pike County, Penn. “Nobody recalls him,” said Lt. Michael McKee of the St. Charles County Department of Corrections.

By this spring, he told friends that he wanted to move closer to his wife’s family, who lived in Brooklyn, and the family moved to Paterson. She was pregnant — this time, with a son, born over the summer. Mr. Saipov started driving for Uber. Still, he was not happy. He told friends and acquaintances that he planned to move back to Uzbekistan. “It’s boring here,” he said. “There’s nothing for me to do here.”

But about a year ago, he began plotting an attack, the authorities said, and he eventually started downloading ISIS videos. Two months ago he decided how he would do it, the way he could inflict maximum damage. It was obvious: a truck.