FBI says investigators have found a second Uzbek man they had been seeking in connection with the attack.
United States President Donald Trump on Thursday demanded death penalty for Uzbek immigrant Sayfullo Saipov after he killed eight people in a truck attack in New York.
Saipov, accused of carrying out the Islamic State-inspired assault on the New York City, was on Thursday charged with terrorism offences by US prosecutors who said he decided to carry out the truck attack 'to inflict maximum damage against civilians'.
Trump has been vocal in condemning the attack by the 29-year-old man, who ploughed a pickup truck down a crowded bike path near the World Trade Centre, killing eight people and injuring nearly a dozen others.
'NYC terrorist was happy as he asked to hang ISIS flag in his hospital room. He killed 8 people, badly injured 12. SHOULD GET DEATH PENALTY!' Trump said in a tweet, in reference to the Federal Bureau of Investigation's statement that Saipov, a resident of New Jersey, 'requested to display ISIS's flag' in his hospital room.
Trump said on Wednesday that it was 'another attack by a very sick and deranged person' while extending his 'thoughts, condolences and prayers' to the victims.
He said he will ask the Congress to end an immigrant visa programme through which Saipov entered the country.
Saipov was charged with providing material support to the Islamic State group and violence and destruction of motor vehicles.
He crashed the truck into a school bus, left the vehicle brandishing imitation firearms. He was shot by police. It was the the worst terrorist attack in on the city since 9/11.
Saipov underwent a surgery in a New York hospital on Wednesday.
He appeared in federal court on Thursday in a wheelchair and did not enter a plea. A 10-page complaint was filed by the FBI before a federal court in New York against him.
The FBI complaint stated that Saipov said he felt good about what he had done.
Saipov told investigators he was inspired by Islamic State videos, in particular one showing their leader Abu Bakr al-Baghdadi, it said.
The suspect decided to conduct a truck attack 'to inflict maximum damage against civilians' and that he specifically chose to strike on Halloween 'because he believed there would be more civilians on the street for the holiday', the complaint says.
He began planning an attack a year ago and decided two months ago to use a truck, officials said.
'In particular, Saipov was motivated to commit the attack after viewing a video in which Abu Bakr al-Baghdadi... questioned what Muslims in the United States and elsewhere were doing to respond to the killing of Muslims in Iraq,' the complaint said.
During a search operation, federal officials obtained two cell phones from his possession. One cellphone contained approximately 90 videos, many of which appear to be, IS-related propaganda.
The cell phone also contained approximately 3,800 images, many of which appear to be IS propaganda. Internet search history of the second cell phone reflects, among other things, that Saipov searched for a store in Passaic in New jersey on October 4, searched for Halloween in New York on October 15, and searched for truck rentals on October 18, the complaint said.
After Saipov rented the truck from Home Depot, he entered the New York City.
In carrying out the attack, Saipov relied on the playbook laid out by ISIS in recent years, officials said.
"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," John Miller, New York Police Department commissioner for intelligence and counterterrorism, said at an earlier news conference.
A handwritten 'document' found near the scene had both Arabic and English text, and included the message that the Islamic State would endure, the complaint said.
In the vicinity of a place where Saipov was shot, he dropped the weapons, which appear to be a paintball gun and pellet gun, and a black bag.
Inside the Bag, law enforcement officers recovered, among other things, three knives and a wallet.
Meanwhile, the FBI said that the investigators have found a second Uzbek man they had been seeking in connection with the attack.
The FBI had released an alert saying they were seeking information about the second man, Mukhammadzoir Kadirov, 32, in connection with the attack.
With ANI inputs.
IMAGE: The white pick-up truck that Saipov rammed into bicyclists and pedestrians after entering the West St-Houston St pedestrian-bike path in Lower Manhattan on Tuesday. Photograph: Reuters