Federal prosecutors on Wednesday brought terrorism charges against the Uzbek immigrant accused in the truck rampage that left eight people dead, saying he carried out the attack in response to the Islamic State group's online calls to action.
Meanwhile, the Federal Bureau of Investigation (FBI) said another person is wanted for questioning in connection with the bloodshed.
Authorities said the driver of the rented Home Depot truck, 29-year-old Sayfullo Saipov, watched ISIS videos on his cellphone and picked Halloween for the attack on a bike lane in lower Manhattan because he knew more people would be out on the streets.
Afterward, as he lay wounded in the hospital, he asked to display the ISIS flag in his room and "stated that he felt good about what he had done," prosecutors said in court papers.
He was charged with providing material support to a terrorist group and committing violence and destruction of motor vehicles.
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Saipov left behind knives and a note, handwritten in Arabic, that included Islamic religious references and said "it will endure", a phrase that commonly refers to ISIS, FBI agent Amber Tyree said in court papers.
Questioned in his hospital bed, Saipov said he had been inspired by ISIS videos and began plotting an attack about a year ago, deciding to use a truck about two months ago, Tyree said. Saipov even rented a truck on October 22 to practise making turns, Tyree said.
John Miller, deputy New York police commissioner for intelligence, said Saipov "appears to have followed, almost exactly to a T, the instructions that ISIS has put out."
In the past few years, the Islamic State has exhorted followers online to use vehicles, knives or other close-at- hand means of killing people in their home countries. England, France and Germany have all seen deadly vehicle attacks since mid-2016.
A November 2016 issue of the group's online magazine detailed features that an attack truck or van should have, suggested renting such a vehicle and recommended targeting crowded streets and outdoor gatherings, according to the SITE Intelligence Group, a militant-monitoring agency.
Carlos Batista, a neighbour of Saipov's in Paterson, New Jersey, said he had seen the suspect and two friends using the same model of rented truck several times in the past three weeks.
It was not clear whether Saipov had been on authorities' radar. Miller said Saipov had never been the subject of a criminal investigation but appears to have links to people who have been investigated.
In yesterday's attack, Saipov drove his speeding truck for nearly a mile along a bike path, running down cyclists and pedestrians, then crashed into a school bus, authorities said. He was shot in the abdomen after he jumped out of the vehicle brandishing two air guns, one in each hand, and yelling "God is great!" in Arabic, they said.
In addition to those killed, 12 people were injured.