Sayfullo Saipov, the main suspect in Tuesday's New York truck attack that killed eight people and injured 12, arrived in the US from Uzbekistan in 2010 and is married with three children.
He became a legal permanent resident of the US through a lottery programme that grants green cards annually to foreign nationals, in an effort to diversify the country's immigrant population.
A day after the attack, Mr Saipov admitted to investigators that he had been inspired by propaganda from so-called Islamic State (IS).
Speaking to the BBC, US-based Uzbek religious activist and blogger Mirrakhmat Muminov, who met Mr Saipov in Ohio soon after he arrived in the US, said the suspect was radicalised online and had become increasingly aggressive.
"He was not well educated and had no knowledge of the Koran before arriving in the US," he said.
"At the beginning of his time here he was a normal sort of person."
But Mr Muminov said that Mr Saipov had become depressed, separated from his community and more resentful and angry after failing to find work as a driver.
"Because of his radical views he frequently used to argue with other Uzbeks and moved to Florida," Mr Muminov said. "From then onwards I lost contact with him."
He had never been the subject of an NYPD or FBI intelligence investigation, according to John Miller, deputy commissioner for the New York Police Department.
However, the New York Times, citing three officials, said the suspect had previously come to the attention of federal authorities via an unrelated probe.
Born in Uzbekistan in February 1988, Mr Saipov emigrated to the US in 2010 after winning a green card via the lottery and is believed to have lived in Ohio, Florida, and New Jersey since.
Mr Muminov said there were about 70,000 people from Uzbekistan now living in the US, with the overwhelming majority in New York City but also smaller populations in Florida - mostly in Orlando - and in Chicago and Ohio.
According to the New York Times, Mr Saipov arrived in the country with a poor command of English and sought work as a truck and Uber taxi driver.
"He was a very good person when I knew him," Uzbek immigrant Kobiljon Matkarov - who met Mr Saipov in Florida several years ago - told the newspaper.
"He liked the US. He seemed very lucky and all the time he was happy and talking like everything is OK. He did not seem like a terrorist, but I did not know him from the inside."
Mr Saipov was shot and injured by a police officer and appeared in court in a wheelchair a day after the attack.
He told investigators he had been planning the attack for a year, and intentionally chose Halloween because he believed there would be more people in the streets.
Authorities found 90 graphic and violent propaganda videos on his phone - one that showed IS leader Abu Bakr al-Baghdadi talking about Muslims avenging deaths in Iraq.
Officials say a note was found in the truck that referred to IS, but New York Governor Andrew Cuomo said it was likely the suspect had acted alone and there was no evidence to suggest a wider plot.
Witnesses said they heard the attacker shout "Allahu Akbar" - Arabic for "God is greatest" - when he emerged from his vehicle after the killings.
Federal prosecutors charged Mr Saipov on two counts: providing material support and resources to IS and violence and destruction of motor vehicles.
President Donald Trump has denounced him as "very sick" and a "deranged person".
He is reported to have been living most recently in Paterson, New Jersey, about 40km (25 miles) north-west of the scene of the attack. The truck involved was rented from nearby Passaic, a former industrial hub just south of Paterson.
About 25,000 to 30,000 Muslims live in the city, the New York Times reported, giving it one of the highest concentrations of Muslims in the New York City area.
Uzbekistan has over the last 20 years taken a hard line against Islamic extremism.
Mr Saipov is not be the first person from the Central Asian country to be accused of plotting terror attacks in the US. Last month a Brooklyn man of Uzbek origin was sentenced to 15 years in prison for plotting terrorist attacks, including threats to kill Barack Obama.