NEW YORK, United States -- The Uzbek immigrant behind New York's worst attack in 16 years confessed to acting in the name of the Islamic State group and "felt good" about the killings, having planned an assault for a year, investigators said Wednesday.
The shocking details emerged after federal agents questioned Sayfullo Saipov in a Manhattan hospital, injured with a police bullet to the abdomen after mowing down pedestrians and cyclists, and colliding with a school bus.
Tuesday's attack saw US President Donald Trump vow an immediate crackdown on the visa program that he said allowed the 29-year-old to immigrate in March 2010.
The attack killed eight people, five of them childhood friends from Argentina celebrating 30 years since their high school graduation, a 31-year-old Belgian mother, and two US men, from New York and neighboring New Jersey.
Of 12 injured, four remained in a critical condition following the deadliest attack in America's financial capital since the Al-Qaeda hijackings on September 11, 2001.
Saipov first started planning an attack a year ago, before settling two months ago on using a truck to kill as many people as possible during Halloween celebrations, according to a federal terrorism complaint.
Prosecutors unveiled the terrorism charges, saying he had waived his rights and confessed to being inspired by IS propaganda, after yelling "Allahu Akbar" ("God is Greatest" in Arabic) upon exiting a rented pickup truck in Lower Manhattan on Tuesday.
"Saipov committed this attack in support of Isis," US acting Manhattan attorney Joon Kim announced.
Saipov was subsequently presented in a wheelchair before a US magistrate, where the charges were formally read in a Manhattan federal court.
He was not required to enter a plea, a spokesman for the US Attorney's office in Manhattan told AFP.
He was found in possession of multiple knives in a black bag, a Florida driving license and two cell phones that contained thousands of IS propaganda images and dozens of IS propaganda videos, Kim said.
"Depicting, among other things, Isis fighters killing prisoners by running over them with a tank, beheading them, and shooting them in the face," Kim added.
"Saipov requested to display Isis's flag in his hospital room and stated that he felt good about what he had done," the charging document revealed.
The complaint listed two counts: provision of material support and resources to a designated foreign terrorist organization, and violence and destruction of motor vehicles. It was not immediately clear if he would face further charges.
Kim said the material support charge carries a maximum sentence of life imprisonment. Federal prosecutors could also potentially seek the death penalty.
In the largely Muslim neighborhood where Saipov lived with his wife and children for little over a year in the New Jersey town of Paterson, residents seethed with anger Wednesday, furious he had besmirched their hard-working immigrant reputation.
"They should hang him!" snapped the manager of a launderette near his modest apartment. "If you come to the US, it's to do something better, not something bad!" she spat, refusing to give her name out of fear.
A cop shot Saipov after he exited the truck brandishing paintball and pellet guns.
Vehicle rammings have been a frequent tactic deployed by IS sympathizers in the West, including in Barcelona, London, Stockholm and in Nice, where a Tunisian suicide truck bomber killed 86 people on Bastille Day last year.
"He appears to have followed almost exactly to a 'T' the instructions that Isis has put out in its social media channels before with instructions to their followers on how to carry out such an attack," senior police officer John Miller said.
New York Governor Andrew Cuomo said Saipov had been radicalized after moving to the United States. He is not a US citizen but a legal permanent resident.
Trump denounced Saipov as an "animal" and said he "would certainly consider" sending him to Guantanamo Bay.
He said he was "starting the process of terminating" the popular green card lottery, which he said had enabled Saipov to enter the country.
"We have to do what's right to protect our citizens," the Republican president told reporters. "We will get rid of this lottery program as soon as possible."
Trump has already slashed the country's annual refugee intake by more than 50 percent, tightened visa issuance around the world and attempted to ban travelers from 11 countries, most of them with Muslim-majority populations, but not Uzbekistan.
Saipov lived in Florida, before moving to Paterson, a former industrial hub about 20 miles (30 kilometers) northeast of New York.
Though preliminary evidence suggests Saipov acted alone, Cuomo has drastically stepped up security at airports, tunnels and Penn Station, which he called the busiest rail hub in the hemisphere.
Authorities announced they had found a second Uzbek they'd been seeking in relation to their investigation -- 32-year-old Mukhammadzoir Kadirov -- but they provided no additional details.
In Manhattan, leaders vowed the annual marathon would go ahead as planned on Sunday. Police said the event, which attracts more than 50,000 runners and 2.5 million spectators, would be the most protected ever.
Uzbekistan, a majority Muslim country that borders Afghanistan and formerly part of the Soviet Union, is a landlocked country racked with poverty, corruption and a stifling authoritarian regime.
Saipov is the fourth Uzbek-linked man in less than a year blamed for deadly strikes overseas, following attacks in Istanbul, St Petersburg and Stockholm. (AFP)