"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. He also made a test run last week.
Investigators work around the wreckage of a Home Depot pickup truck a day after it was used in a terror attack in NY on November 1, 2017.
Investigators said they believe Saipov was a "lone wolf" when he drove a rented Home Depot pickup truck into bicyclists and pedestrians on Tuesday afternoon.
Saipov appeared in a wheelchair before a U.S. magistrate in Manhattan federal court, where the charges were formally read.
Saipov did waive his right to remain silent or have an attorney when he agreed to speak to investigators from his bed at Bellevue Hospital Center in Manhattan, where he was being treated after being shot by a police officer, according to the criminal complaint.
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. Prosecutors announced federal terrorism charges against Saipov late Wednesday, which means he can get the death penalty if convicted - even though NY state has no death penalty.More news: Texas church is scene of shooting
During the last few weeks, Saipov searched the internet for information on Halloween in New York City and rented a truck to practice turns.
The complaint listed two counts: provision of material support and resources to a designated foreign terrorist organization, and violence and destruction of motor vehicles.
SHOULD GET DEATH PENALTY!' the President tweeted Wednesday night. "Every time they hit us, we know it is ISIS, we hit them like you won't believe", he said. He repeated the call Thursday, saying he'd love to send the "NYC terrorist" to Guantanamo, but that process would take longer than going through the federal system. The vehicle charge raises the possibility that he will face the death penalty. BEFORE THE ATTACKOfficials say Saipov entered the United States through JFK Airport from Uzbekistan in 2010.
The complaint also said that Saipov specifically chose Halloween for the attack date because he thought "there would be more civilians on the street".
Of those killed on Tuesday, five were Argentine tourists, who were among a group of friends visiting NY to celebrate the 30th anniversary of their high school graduation, one was a Belgian citizen, one was a NY resident and one lived in New Jersey.